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/serializers.py

@@ -122,6 +122,11 @@ class PromptSerializer(serializers.ModelSerializer):
             "id",
         )
 
+    def validate_message(self, value: str) -> str:
+        if value is None or not str(value).strip():
+            raise serializers.ValidationError("Message text cannot be empty.")
+        return str(value).strip()
+
 
 class BasicUserSerializer(serializers.ModelSerializer):
     class Meta: