Add TypeScript types for api

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

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

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

Start by locating the API handlers and the existing error-construction paths; the issue does not name specific files or tests. Determine how API types are currently represented and where handler errors are surfaced. Done means the API has TypeScript types and handler failures provide more descriptive messages, with relevant tests or checks passing.

Written by the indexing model from the issue text.