refactor(tools): enhance type annotations and clean up variable usage

- Changed `VariableKey` to inherit from `str` and `Enum` for better compatibility.
- Updated the type annotation of `meta` in `create_blob_message` to use more modern syntax.
- Removed unnecessary `.value` in `save_as` parameter for cleaner code.

api/core/tools/provider/builtin/dalle/tools/dalle3.py

@@ -66,7 +66,7 @@ def _invoke(
         for image in response.data:
             mime_type, blob_image = DallE3Tool._decode_image(image.b64_json)
             blob_message = self.create_blob_message(
-                blob=blob_image, meta={"mime_type": mime_type}, save_as=self.VariableKey.IMAGE.value
+                blob=blob_image, meta={"mime_type": mime_type}, save_as=self.VariableKey.IMAGE
             )
             result.append(blob_message)
         return result

api/core/tools/tool/tool.py

@@ -62,7 +62,7 @@ def __init__(self, **data: Any):
     def __init__(self, **data: Any):
         super().__init__(**data)
 
-    class VariableKey(Enum):
+    class VariableKey(str, Enum):
         IMAGE = "image"
 
     def fork_tool_runtime(self, runtime: dict[str, Any]) -> "Tool":