fix: V3 migration issues - Auto-migrate old sessions and use Mistral for chat names

- Fix integrity check to auto-migrate old MongoDB messages to S3
- Clear MongoDB messages array after successful migration
- Initialize S3 for new empty sessions
- Change chat name model from Claude to Mistral (V3 compliance)
- Resolves session integrity failure on second message
- Resolves Bedrock ResourceNotFoundException for chat names

api_routes_v2.py

@@ -142,12 +142,20 @@ def _validate_conversation_integrity(session_id: str) -> Dict[str, Any]:
             session_manager.update_session(session_id, {"conversation_s3_key": default_key})
             return {"valid": True, "note": "Fixed missing s3_key metadata"}
         except ClientError:
-            # S3 missing. If session has messages in MongoDB, we should migrate them?
-            # For strict V3, we error if S3 missing, unless session is empty.
+            # S3 missing. Migrate old MongoDB messages to S3 (V3 migration path)
             msgs = session.get("messages", [])
             if msgs:
-                return {"valid": False, "error": "CORRUPTED: Session has messages in MongoDB but S3 file is missing"}
-            return {"valid": True, "note": "New/Empty session"}
+                # Auto-migrate old session to V3
+                try:
+                    _save_conversation_to_s3(session_id, msgs)
+                    # Clear old MongoDB messages after successful migration
+                    session_manager.update_session(session_id, {"messages": []})
+                    return {"valid": True, "note": "Migrated old MongoDB messages to S3"}
+                except Exception as e:
+                    return {"valid": False, "error": f"Migration failed: {str(e)}"}
+            # New empty session - initialize S3
+            _save_conversation_to_s3(session_id, [])
+            return {"valid": True, "note": "Initialized new session"}
 
     try:
         s3.head_object(Bucket=S3_BUCKET, Key=key)
@@ -357,7 +365,8 @@ def _resolve_bedrock_model_for_titles(session: Dict[str, Any]) -> str:
     except Exception:
         model = None
     if not model:
-        model = os.getenv("BEDROCK_MODEL_ID") or os.getenv("BEDROCK_DEFAULT_MODEL") or "anthropic.claude-3-haiku-20240307"
+        # V3 RULE: NO CLAUDE - Use Mistral or Titan only
+        model = os.getenv("BEDROCK_MODEL_ID") or os.getenv("BEDROCK_DEFAULT_MODEL") or "mistral.mistral-large-2402-v1:0"
     return model