Add unit tests for middleware

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

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

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

No files or existing tests are named. Start by locating the middleware and model error-handling paths, then identify the current error-message behavior and its test entry points. Done means adding unit-test coverage for the middleware and making the relevant errors more descriptive, with the test suite passing.

Written by the indexing model from the issue text.