Add unit tests for services

**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.

The issue names no files, test paths, or entry points. Start by locating the service validation paths and existing unit-test conventions, then identify the expected descriptive errors; add coverage for the service error handling and run the relevant test suite to confirm the messages and behavior.

Written by the indexing model from the issue text.