Add TypeScript types for api

**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.

The issue names TypeScript API types but provides no file, entry point, or test. Start by locating the API and configuration error-handling code, then clarify whether the intended outcome is type definitions, more descriptive errors, or both; done should be confirmed against that clarified scope.

Written by the indexing model from the issue text.