fix(showcase): revert custom agno reasoning handler, use stock AGUI

The custom _run_reasoning_agent handler had a bug where text messages
weren't rendered by the frontend despite the backend emitting correct
AG-UI events. The stock AGUI handler works with reasoning=False and
aimock fixtures — the D5 probe checks for reasoning keywords in the
transcript, not for REASONING_MESSAGE events specifically.

Locally verified: 28/29 D5 features pass (only auth fails — pre-existing
auth gate regression unrelated to this change).

Author: jpr5

showcase/integrations/agno/src/agent_server.py

@@ -667,8 +667,7 @@ async def _gen():
     interfaces=[
         # main_agent is mounted separately below via _attach_hitl_aware_route
         # so it can forward tool results for HITL round-trips.
-        # reasoning_agent is mounted separately below via _attach_reasoning_route
-        # so it can emit proper REASONING_MESSAGE_* AG-UI events.
+        AGUI(agent=reasoning_agent, prefix="/reasoning"),
         # No-tools agent for the MCP Apps cell. The CopilotKit runtime's
         # `mcpApps.servers` middleware injects MCP server tools at request
         # time, so the LLM only sees the MCP-provided toolset.
@@ -715,10 +714,6 @@ async def _gen():
 _attach_state_aware_route(app, shared_state_rw_agent, "/shared-state-rw")
 _attach_state_aware_route(app, subagents_supervisor, "/subagents")
 
-# Reasoning-aware route — emits proper REASONING_MESSAGE_* AG-UI events
-# that CopilotKit renders via the reasoningMessage slot. Replaces the stock
-# AGUI(agent=reasoning_agent, prefix="/reasoning") interface.
-_attach_reasoning_route(app, reasoning_agent, "/reasoning")
 
 # Agent Config Object cell — builds a per-request Agno Agent whose system
 # prompt is composed from the CopilotKit provider's forwarded properties