fix: handle non-JSON LLM API responses gracefully

- Check resp.content_type before calling resp.json()
- Return clear error if server returns HTML (e.g. proxy error or wrong endpoint)
- Prevents 'Attempt to decode JSON with unexpected mimetype' crashes

harnessed_agent/llm_client.py

@@ -229,6 +229,19 @@ async def _post_chat_completions(
                             }
                         }
 
+                    # Check content type before parsing JSON
+                    content_type = resp.content_type
+                    if 'json' not in content_type:
+                        err_text = await resp.text()
+                        return {
+                            'error': {
+                                'message': f'LLM API returned non-JSON response (Content-Type: {content_type}). URL may be incorrect or blocked.',
+                                'type': 'content_type_error',
+                                'code': resp.status,
+                                'detail': err_text[:500],
+                            }
+                        }
+
                     return await resp.json()
 
         except asyncio.TimeoutError: