Ignore WS heartbeats and reject empty chat messages (#32)

## Summary
- Closes #31
- Ignore WebSocket `type: ping` heartbeats so keepalives no longer create conversations or hit title/LLM pipelines
- Reject empty/whitespace user messages in both chat consumers, `PromptSerializer`, and REST conversation prompt POST

## Test plan
- [x] `UserPromptGuardTestCase`, `PromptSerializerTestCase` blank/whitespace cases
- [x] `WebSocketReceiveGuardTestCase` ping ignore + empty message rejection (both WS routes)
- [ ] Deploy to beta; leave idle tab open and confirm no new rogue conversations
- [ ] Confirm normal chat send still works

Related FE: https://git.aimloperations.com/ai_ml_operations/chat_web_app/issues/51Reviewed-on: #32

llm_be/chat_backend/consumers_graph.py

@@ -22,6 +22,7 @@ from .services.title_generator import title_generator
 from .services.moderation_classifier import moderation_classifier, ModerationLabel
 from .services.prompt_classifier.prompt_classifier import PromptClassifier, PromptType
 from .services.data_analysis_service import AsyncDataAnalysisService
+from .utils import has_usable_user_prompt, is_heartbeat_payload, normalize_user_message
 
 logger = logging.getLogger(__name__)
 
@@ -288,13 +289,29 @@ class ChatConsumerGraph(AsyncWebsocketConsumer):
         print("Text Data: ", text_data)
         if text_data:
             data = json.loads(text_data)
+            # Keepalive frames must not create conversations or hit the LLM.
+            if is_heartbeat_payload(data):
+                return
+
             model = data.get("modelName", "Turbo")
-            message = data.get("message", None)
+            message = normalize_user_message(data.get("message", None))
             conversation_id = data.get("conversation_id", None)
             email = data.get("email", None)
             file = data.get("file", None)
             file_type = data.get("fileType", "")
-            
+
+            if not has_usable_user_prompt(message, file):
+                logger.info("Ignoring websocket payload with empty message")
+                await self.send_json_message(
+                    json.dumps(
+                        {
+                            "type": "error",
+                            "content": "Message text cannot be empty.",
+                        }
+                    )
+                )
+                return
+
             if not conversation_id:
                 title = await title_generator.generate_async(message)
                 conversation_id = await create_conversation(message, email, title)