Add TypeScript types 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 does not identify files, tests, or an entry point. Clarify which authentication service and error paths are in scope, then define the expected TypeScript types and error-message behavior and add corresponding tests.

Written by the indexing model from the issue text.