Add TypeScript types for models

**Problem**
When working with auth, 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 name any files, tests, or entry points. Start by locating the authentication models and current error-handling code, then determine how TypeScript types fit those models. Done means auth failures expose more descriptive error messages and the relevant behavior is covered by verification.

Written by the indexing model from the issue text.