fix: preserve reasoning_content in multi-turn conversations

When using thinking-enabled models (like Kimi K2.5, DeepSeek), the API
expects reasoning_content on all assistant messages. The original
ChatDeepSeek stores reasoning_content in additional_kwargs but doesn't
include it when making subsequent API calls, causing "reasoning_content
is missing" errors.

This adds PatchedChatDeepSeek which overrides _get_request_payload to
restore reasoning_content from additional_kwargs into the payload.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/debug.py

@@ -41,7 +41,7 @@ async def main():
             "thinking_enabled": True,
             "is_plan_mode": True,
             # Uncomment to use a specific model
-            "model_name": "deepseek-v3.2",
+            "model_name": "kimi-k2.5",
         }
     }