Add TypeScript types for auth

**Problem**
When working with config, 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 file, test, or entry point is identified. Start by locating the authentication configuration and existing error-handling code, then confirm the expected TypeScript types and error cases with the issue context; done means auth configuration has types and failures produce more descriptive messages.

Written by the indexing model from the issue text.