feat(showcase/google-adk): port langgraph-python demos to fill dashboard column (CopilotKit#4318)

## Summary

Brings the **Google ADK** column on
https://dashboard.showcase.copilotkit.ai/#coverage from 8 wired cells
(parity_tier `minimal`) up to **35 wired cells** (parity_tier `partial`)
— using `langgraph-python` (38/39) as the reference. All ports use real
Google ADK + Gemini 2.5 Flash agents wired through `ag-ui-adk`; nothing
is stubbed.

### Backend

- `src/agent_server.py` refactored from a single shared `LlmAgent`
mounted at `/` to one `ADKAgent` per demo at `/<agent_name>`, driven by
a new `src/agents/registry.py` source-of-truth (with an `AgentSpec`
dataclass for `predict_state` / `emit_messages_snapshot` kwargs).
- 18 new per-demo agent modules covering reasoning (Gemini thinking
mode), tool-rendering (4 variants), HITL (in-chat + in-app),
shared-state (read-write + per-token streaming via
`PredictStateMapping`), multi-agent supervisor, A2UI dynamic +
fixed-schema, BYOC, open-gen-ui (basic + advanced), agent-config,
readonly-context, multimodal, beautiful-chat, plus the existing
recipe-themed shared-state-read.

### Frontend

- `src/app/api/copilotkit/route.ts` now maps each agent name to its
matching backend path.
- 6 dedicated runtime routes for demos that need bespoke runtime config:
- `copilotkit-auth` — V2 `createCopilotRuntimeHandler` with an
`onRequest` 401 gate
- `copilotkit-declarative-gen-ui` and `copilotkit-a2ui-fixed-schema` —
`a2ui.injectA2UITool: false` because the agents own their own A2UI tools
- `copilotkit-byoc-hashbrown`, `copilotkit-byoc-json-render`,
`copilotkit-ogui` (shared by open-gen-ui + advanced)
- 26 new / replaced demo pages with their helper components.

### Manifest

`showcase/packages/google-adk/manifest.yaml` declares 35 features (was
8). `pnpm generate-registry` validates clean.

### Skipped (intentional)

- `gen-ui-interrupt`, `interrupt-headless` — depend on LangGraph's
`interrupt()` graph primitive; ADK has no direct equivalent. The
high-level `useHumanInTheLoop` case is covered via `hitl-in-chat`.
- `mcp-apps` — `ag_ui_adk` does support `MCPToolset`, but a working
MCP-driven UI demo is a separate chunk; left for follow-up.

## Test plan

- [x] `pnpm generate-registry` validates manifest end-to-end (Catalog:
reference = langgraph-python with 37 wired features; google-adk now at
35/39 wired)
- [x] `npx tsc --noEmit` in `showcase/packages/google-adk` — no new
errors (pre-existing `byoc-json-render/registry.tsx` and
`shared-state-write/page.tsx` errors untouched)
- [x] Python syntax: `ast.parse` over every file in `src/agents/` — all
parse cleanly
- [ ] Local smoke test: `pnpm dev` in `showcase/packages/google-adk`,
hit each new `/demos/*` route, verify L2/L3 health
- [ ] Railway deploy: confirm health probes go green and dashboard
column lights up

## Caveats worth knowing before deploy

1. **Per-token streaming** in `shared-state-streaming` uses
`PredictStateMapping(stream_tool_call=True)`. Per `ag_ui_adk` docs, true
per-token argument streaming requires Gemini 3+ via Vertex AI; on Gemini
2.5 Flash this should still work but at chunk-level granularity.
2. **Subagents** uses direct `genai.Client.models.generate_content`
calls inside each delegation tool, not ADK's native `sub_agents` /
`AgentTool`. Functionally equivalent for the demo; a follow-up could
refactor to ADK-native multi-agent.
3. **A2UI fixed-schema** has an inline `_build_flight_operations` helper
instead of importing `copilotkit.a2ui`. The `copilotkit` Python SDK
isn't in `requirements.txt` for this package — adding it later and
refactoring is straightforward.
4. **`/agents/state` collision**: `ag_ui_adk.add_adk_fastapi_endpoint`
hardcodes `POST /agents/state` regardless of the `path` arg. With N
agents mounted, only the first registered agent's state endpoint is
reachable. Most demos don't use it; `headless-complete` could.
5. **Beautiful-chat** is intentionally minimal vs. the langgraph-python
sibling's richer canvas/layout split.

Author: jpr5

showcase/packages/google-adk/docs-links.json

@@ -3,41 +3,149 @@
   "features": {
     "agentic-chat": {
       "og_docs_url": "https://docs.copilotkit.ai/adk",
-      "shell_docs_path": "/prebuilt-components"
+      "shell_docs_path": null
     },
-    "hitl-in-chat": {
-      "og_docs_url": "https://docs.copilotkit.ai/adk/human-in-the-loop",
-      "shell_docs_path": "/generative-ui/your-components/interactive"
+    "beautiful-chat": {
+      "og_docs_url": "https://docs.copilotkit.ai/adk",
+      "shell_docs_path": null
     },
-    "tool-rendering": {
-      "og_docs_url": "https://docs.copilotkit.ai/adk/generative-ui/tool-rendering",
-      "shell_docs_path": "/generative-ui/tool-rendering"
+    "cli-start": {
+      "og_docs_url": "https://docs.copilotkit.ai/adk/quickstart",
+      "shell_docs_path": null
+    },
+    "prebuilt-sidebar": {
+      "og_docs_url": "https://docs.copilotkit.ai/adk",
+      "shell_docs_path": null
+    },
+    "prebuilt-popup": {
+      "og_docs_url": "https://docs.copilotkit.ai/adk",
+      "shell_docs_path": null
+    },
+    "chat-slots": {
+      "og_docs_url": "https://docs.copilotkit.ai/adk",
+      "shell_docs_path": null
+    },
+    "chat-customization-css": {
+      "og_docs_url": null,
+      "shell_docs_path": null
+    },
+    "headless-simple": {
+      "og_docs_url": "https://docs.copilotkit.ai/adk",
+      "shell_docs_path": null
+    },
+    "headless-complete": {
+      "og_docs_url": "https://docs.copilotkit.ai/adk",
+      "shell_docs_path": null
+    },
+    "agentic-chat-reasoning": {
+      "og_docs_url": "https://docs.copilotkit.ai/custom-look-and-feel/reasoning-messages",
+      "shell_docs_path": null
+    },
+    "reasoning-default-render": {
+      "og_docs_url": "https://docs.copilotkit.ai/custom-look-and-feel/reasoning-messages",
+      "shell_docs_path": null
+    },
+    "frontend-tools": {
+      "og_docs_url": "https://docs.copilotkit.ai/adk/frontend-tools",
+      "shell_docs_path": null
+    },
+    "frontend-tools-async": {
+      "og_docs_url": "https://docs.copilotkit.ai/adk/frontend-tools",
+      "shell_docs_path": null
     },
     "gen-ui-tool-based": {
       "og_docs_url": "https://docs.copilotkit.ai/adk/generative-ui/your-components/display-only",
-      "shell_docs_path": "/generative-ui/your-components/display-only"
+      "shell_docs_path": null
+    },
+    "hitl-in-chat": {
+      "og_docs_url": "https://docs.copilotkit.ai/adk/human-in-the-loop",
+      "shell_docs_path": null
+    },
+    "hitl-in-app": {
+      "og_docs_url": "https://docs.copilotkit.ai/adk/frontend-tools",
+      "shell_docs_path": null
+    },
+    "declarative-gen-ui": {
+      "og_docs_url": "https://docs.copilotkit.ai/adk/generative-ui/a2ui/dynamic-schema",
+      "shell_docs_path": null
+    },
+    "a2ui-fixed-schema": {
+      "og_docs_url": "https://docs.copilotkit.ai/adk/generative-ui/a2ui/fixed-schema",
+      "shell_docs_path": null
+    },
+    "open-gen-ui": {
+      "og_docs_url": "https://docs.copilotkit.ai/generative-ui/open-generative-ui",
+      "shell_docs_path": null
+    },
+    "open-gen-ui-advanced": {
+      "og_docs_url": "https://docs.copilotkit.ai/generative-ui/open-generative-ui",
+      "shell_docs_path": null
     },
     "gen-ui-agent": {
       "og_docs_url": "https://docs.copilotkit.ai/adk/generative-ui/state-rendering",
-      "shell_docs_path": "/generative-ui/state-rendering"
+      "shell_docs_path": null
+    },
+    "tool-rendering-default-catchall": {
+      "og_docs_url": "https://docs.copilotkit.ai/adk/generative-ui/tool-rendering",
+      "shell_docs_path": null
+    },
+    "tool-rendering-custom-catchall": {
+      "og_docs_url": "https://docs.copilotkit.ai/adk/generative-ui/tool-rendering",
+      "shell_docs_path": null
+    },
+    "tool-rendering": {
+      "og_docs_url": "https://docs.copilotkit.ai/adk/generative-ui/tool-rendering",
+      "shell_docs_path": null
+    },
+    "tool-rendering-reasoning-chain": {
+      "og_docs_url": "https://docs.copilotkit.ai/adk/generative-ui/tool-rendering",
+      "shell_docs_path": null
     },
     "shared-state-read-write": {
       "og_docs_url": "https://docs.copilotkit.ai/adk/shared-state/in-app-agent-write",
-      "shell_docs_path": "/shared-state"
+      "shell_docs_path": null
     },
     "shared-state-streaming": {
       "og_docs_url": "https://docs.copilotkit.ai/adk/shared-state/predictive-state-updates",
-      "shell_docs_path": "/shared-state"
+      "shell_docs_path": null
+    },
+    "readonly-state-agent-context": {
+      "og_docs_url": "https://docs.copilotkit.ai/adk/shared-state/in-app-agent-read",
+      "shell_docs_path": null
     },
     "subagents": {
+      "og_docs_url": null,
+      "shell_docs_path": null
+    },
+    "voice": {
       "og_docs_url": "https://docs.copilotkit.ai/adk",
-      "shell_docs_path": "/multi-agent/subagents"
+      "shell_docs_path": null
+    },
+    "multimodal": {
+      "og_docs_url": null,
+      "shell_docs_path": null
+    },
+    "auth": {
+      "og_docs_url": "https://docs.copilotkit.ai/adk",
+      "shell_docs_path": null
+    },
+    "agent-config": {
+      "og_docs_url": null,
+      "shell_docs_path": null
+    },
+    "byoc-hashbrown": {
+      "og_docs_url": null,
+      "shell_docs_path": null
+    },
+    "byoc-json-render": {
+      "og_docs_url": null,
+      "shell_docs_path": null
     }
   },
   "missing": [
     {
-      "feature": "subagents",
-      "reason": "No /adk/multi-agent or /adk/subagents page on docs.copilotkit.ai (SPA fallback). OG falls back to the adk framework root; shell points at the unified /multi-agent/subagents content."
+      "feature": "all",
+      "reason": "4085 shell does not have a google-adk-scoped docs tree. Shell paths set to null rather than linking to framework-agnostic pages that would be misleading under the google-adk URL. OG URLs point at the real /adk/... pages on docs.copilotkit.ai where they exist; null where there's no canonical ADK page (subagents, multimodal, agent-config, byoc-*, chat-customization-css)."
     }
   ]
 }