Fix issue with keyerror in list comprehension (microsoft#775)

Co-authored-by: Abby Hartman <[email protected]>

app.py

@@ -848,7 +848,12 @@ async def promptflow_request(request):
 
 
 async def send_chat_request(request):
-    filtered_messages = [message for message in request['messages'] if message['role'] != 'tool']
+    filtered_messages = []
+    messages = request.get("messages", [])
+    for message in messages:
+        if message.get("role") != 'tool':
+            filtered_messages.append(message)
+
     request['messages'] = filtered_messages
     model_args = prepare_model_args(request)