Add TypeScript types for handlers

**Problem**
When working with routes, 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 route handlers but provides no files, tests, or entry points. Start by locating the route handler definitions and existing error-handling paths, then clarify the expected TypeScript types and error-message wording; done means the requested handler types and more descriptive route errors are covered.

Written by the indexing model from the issue text.