Fix action execution server JSONResponse (#7721)

Author: li-boxuan

openhands/runtime/action_execution_server.py

@@ -585,7 +585,10 @@ async def validation_exception_handler(
         logger.error(f'Validation error occurred: {exc}')
         return JSONResponse(
             status_code=422,
-            content={'detail': 'Invalid request parameters', 'errors': exc.errors()},
+            content={
+                'detail': 'Invalid request parameters',
+                'errors': str(exc.errors()),
+            },
         )
 
     @app.middleware('http')