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/models/config.py

@@ -2,6 +2,7 @@
 
 import os
 from pathlib import Path
+from typing import Any
 
 import yaml
 from pydantic import BaseModel, Field, model_validator
@@ -19,6 +20,10 @@ class LLMConfig(BaseModel):
     max_retries: int = Field(default=3, description="Max retry attempts on transient errors")
     retry_backoff_base: float = Field(default=1.0, description="Base seconds for exponential backoff")
     retry_backoff_max: float = Field(default=30.0, description="Maximum backoff seconds")
+    extra_body: dict[str, Any] = Field(
+        default_factory=dict,
+        description="Extra parameters merged into the API request body (model-specific)",
+    )
 
 
 class AgentConfig(BaseModel):