fix(mcp): fix stale patch target in auth tests and update stale docstring

- Remove mock_sm.find_user_with_relationships.return_value = None from
  _mock_sm_ctx: load_user_with_relationships delegates to the global
  security_manager (not app.appbuilder.sm), so setting it on mock_sm had
  no effect and broke MagicMock(spec=[]) tests.
- Add _patch_load_user_not_found() helper that patches
  superset.mcp_service.auth.load_user_with_relationships directly.
- Apply it to the 3 JWT-path tests that expect ValueError("not found"):
  test_jwt_access_token_skips_api_key_auth,
  test_namespaced_claim_without_api_key_client_id_is_ignored,
  test_unnamespaced_passthrough_claim_does_not_trigger_api_key_path.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>