feat: add extra_body config for model-specific API parameters

Allows passing arbitrary parameters (e.g., enable_thinking, reasoning_effort)
to the LLM API request body via config.yaml, solving reasoning-only response
loops with models like Qwen 3.x without requiring code changes per model.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/services/llm.py

@@ -151,6 +151,10 @@ class LLMClient:
         if tools:
             payload["tools"] = tools
 
+        # Merge model-specific extra parameters (e.g., enable_thinking, reasoning_effort)
+        if self._config.extra_body:
+            payload.update(self._config.extra_body)
+
         try:
             async with self._client.stream(
                 "POST", self._config.api_path, json=payload