Add unit tests for validation

**Problem**
When working with store, 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, or validation entry point is named. Start by locating the store validation code and its existing tests, then identify the current error paths and expected wording. Done means the validation behavior has focused unit-test coverage and produces more descriptive errors.

Written by the indexing model from the issue text.