fix RetrievalMethod StrEnum (#26768)

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
Co-authored-by: crazywoola <100913391+crazywoola@users.noreply.github.com>

api/core/rag/entities/event.py

@@ -1,11 +1,11 @@
 from collections.abc import Mapping
-from enum import Enum
+from enum import StrEnum
 from typing import Any
 
 from pydantic import BaseModel, Field
 
 
-class DatasourceStreamEvent(Enum):
+class DatasourceStreamEvent(StrEnum):
     """
     Datasource Stream event
     """