Add unit tests for auth

**Problem**
When working with services, the error messages could be more descriptive.

**Suggestion**
Improve error handling and add detailed error messages.

**Context**
This would help with debugging and maintenance.

The issue names no files, entry points, or existing tests. Start by locating the authentication service and its current error-handling tests; define completion as unit coverage for the auth paths and clearer error messages that provide useful debugging context.

Written by the indexing model from the issue text.