refactor: centralize default end user session ID constant (#25416)

This PR refactors the handling of the default end user session ID by centralizing it as an enum in the models module where the `EndUser` model is defined. This improves code organization and makes the relationship between the constant and the model clearer.

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: crazywoola <100913391+crazywoola@users.noreply.github.com>

api/controllers/files/upload.py

@@ -86,7 +86,7 @@ class PluginUploadFileApi(Resource):
             filename=filename,
             mimetype=mimetype,
             tenant_id=tenant_id,
-            user_id=user_id,
+            user_id=user.id,
             timestamp=timestamp,
             nonce=nonce,
             sign=sign,