Add unit tests for api

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

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

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

Locate the API configuration path and any existing unit-test entry points, then read how configuration errors are currently produced. Clarify which cases need coverage and what details each error should include; done means the API tests cover those cases and the resulting messages are more descriptive.

Written by the indexing model from the issue text.