Add unit tests for auth

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

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

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

No files, tests, or entry points are named. Start by locating the authentication and API error-handling code and the existing unit-test setup. Done should mean that the relevant error paths have more descriptive messages and unit tests cover them, but the issue needs concrete cases and acceptance criteria first.

Written by the indexing model from the issue text.