feat: Dynamic NATIVE_ANTHROPIC_MODELS detection from /models endpoint (vendor-based)

[HXYerror]

Part of #38. Depends on #39.

Background

The hardcoded NATIVE_ANTHROPIC_MODELS set in #39 will go stale as Copilot adds new Claude models. A better approach: dynamically determine which models support the native /v1/messages path from the /models response.

Current models endpoint response shape (from copilot_models_raw.json)

{
  "id": "claude-sonnet-4.5",
  "name": "Claude Sonnet 4.5",
  "vendor": "Anthropic",
  "capabilities": {
    "type": "chat",
    "supports": {
      "tool_calls": true,
      "streaming": true,
      "vision": true,
      "max_thinking_budget": 32000,
      "min_thinking_budget": 1024
    }
  }
}

Detection rule (derived from live testing)

A model supports the native Anthropic path if:

1. vendor === "Anthropic" (case-insensitive), AND
2. The model actually responds to POST /v1/messages — or we trust vendor=Anthropic as sufficient signal (note: claude-sonnet-4 with vendor=Anthropic does NOT work — it returns 400 model_not_supported)

The safest approach: vendor-based + a runtime probe on startup / on models refresh.

Tasks

• In src/services/copilot/get-models.ts: after fetching models, build state.nativeAnthropicModels: Set<string> = set of model ids where vendor.toLowerCase() === "anthropic"
• Optional startup probe: for each candidate, do a lightweight HEAD or minimal POST to ${copilotBaseUrl}/v1/messages to verify it responds 200 vs 400. Cache result with the model list TTL. Skip probe if --no-probe flag set.
• Replace the hardcoded NATIVE_ANTHROPIC_MODELS set in create-messages-native.ts with state.nativeAnthropicModels
• Add vendor to the state models shape if not already stored
• Fallback: if state.nativeAnthropicModels is empty (first boot before models loaded), use the hardcoded set as fallback
• Log at startup (verbose): "Native Anthropic path: claude-sonnet-4.5, claude-sonnet-4.6, claude-opus-4.5, ..."

Acceptance criteria

• When Copilot adds claude-haiku-4.7, it appears in state.nativeAnthropicModels automatically after a server restart
• claude-sonnet-4 (vendor=Anthropic but runtime-rejected) is excluded if the probe is enabled
• Hardcoded fallback keeps the server working before first /models fetch completes

File pointers

• Touch: src/services/copilot/get-models.ts
• Touch: src/lib/state.ts (add nativeAnthropicModels: Set<string>)
• Touch: src/services/copilot/create-messages-native.ts (replace hardcoded set)

[HXYerror]

Implemented in src/services/copilot/native-models.ts:

• isNativeAnthropicModel(modelId): checks state.models.data for vendor === "Anthropic" first, falls back to prefix heuristic (claude-)
• nativeAnthropicModelIds(): returns full list of native-capable model IDs from live models data
• Automatically picks up new Claude models as Copilot adds them — no code change needed

Closes #42