Add TypeScript types for middleware

**Problem**
When working with core, 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 mentions middleware types and more descriptive core error messages but names no files, tests, or entry points. Start by locating the middleware type definitions and core error-handling code, then clarify the expected types and message behavior. Done should be defined by agreed requirements and tests covering them.

Written by the indexing model from the issue text.