Address review feedback from PR #33140

- Keep explicit error when API context provider exists but v1 is not
  available, only return empty holder when there is no provider at all
- Update tests in core-app-api to expect the new error message when
  rendering outside a provider
- Add @backstage/core-plugin-api to changeset since it re-exports
  useApiHolder

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Made-with: Cursor
Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

packages/frontend-plugin-api/src/apis/system/useApi.tsx

@@ -32,7 +32,11 @@ export function useApiHolder(): ApiHolder {
     return emptyApiHolder;
   }
 
-  return versionedHolder.atVersion(1) ?? emptyApiHolder;
+  const apiHolder = versionedHolder.atVersion(1);
+  if (!apiHolder) {
+    throw new NotImplementedError('ApiContext v1 not available');
+  }
+  return apiHolder;
 }
 
 /**