fix: Can not blank conversation ID validation in chat payloads (#29436)

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

api/tests/unit_tests/controllers/service_api/app/test_chat_request_payload.py

@@ -0,0 +1,25 @@
+import uuid
+
+import pytest
+from pydantic import ValidationError
+
+from controllers.service_api.app.completion import ChatRequestPayload
+
+
+def test_chat_request_payload_accepts_blank_conversation_id():
+    payload = ChatRequestPayload.model_validate({"inputs": {}, "query": "hello", "conversation_id": ""})
+
+    assert payload.conversation_id is None
+
+
+def test_chat_request_payload_validates_uuid():
+    conversation_id = str(uuid.uuid4())
+
+    payload = ChatRequestPayload.model_validate({"inputs": {}, "query": "hello", "conversation_id": conversation_id})
+
+    assert payload.conversation_id == conversation_id
+
+
+def test_chat_request_payload_rejects_invalid_uuid():
+    with pytest.raises(ValidationError):
+        ChatRequestPayload.model_validate({"inputs": {}, "query": "hello", "conversation_id": "invalid"})