Add TypeScript types for middleware

**Problem**
When working with data, 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 but does not identify a file, test, entry point, or specific type surface. First locate the middleware implementation and existing TypeScript declarations, then clarify whether the goal is middleware types, more descriptive data-related errors, or both; done should be defined by the agreed type coverage and validation tests.

Written by the indexing model from the issue text.