Add TypeScript types for middleware

**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.

The issue names middleware and handlers but no files, tests, or entry points. Start by locating the middleware implementation and handler entry points, then clarify whether the goal is TypeScript types or more descriptive error messages. Done should be defined by the confirmed scope and corresponding type or error-handling tests.

Written by the indexing model from the issue text.