Add unit tests for core

**Problem**
When working with validation, 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 file, test entry point, or validation path is identified in the issue. Begin by locating the core validation code and existing unit-test setup; document the expected detailed error messages, then confirm the tests cover the cases and pass.

Written by the indexing model from the issue text.