fixed defaults for type error task summary

agent_monitor/monitor.py

@@ -91,10 +91,10 @@ async def analyze_task(calls, llm_client):
     except Exception as e:
         print(f"Error in task summarization: {str(e)}")
         return TaskSummary(
-            overview="Summary generation failed",
-            key_successes=[],
-            main_challenges=[],
-            overall_assessment="Unable to assess due to error"
+            overview="Not available",
+            key_successes='Not available',
+            main_challenges='Not available',
+            overall_assessment="Not available"
         )
 
 async def analyze_step(call, step_number, total_steps, llm_client):