fix: serialize datetime objects before JSON encoding in create_agent_execution

dashboard/backend/db/opc_db.py

@@ -537,8 +537,14 @@ async def create_agent_execution(
             raise ValueError("Task or agent not found")
 
         # Build input context
+        # Prepare input context with serialized datetimes
+        task_for_context = dict(task)
+        for key in ["created_at", "updated_at", "completed_at", "due_date"]:
+            if task_for_context.get(key):
+                task_for_context[key] = task_for_context[key].isoformat() if hasattr(task_for_context[key], 'isoformat') else task_for_context[key]
+
         input_context = {
-            "task": dict(task),
+            "task": task_for_context,
             "agent": dict(agent),
             "timestamp": datetime.utcnow().isoformat()
         }