Add unit tests for core

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

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

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

Start by locating the core parser implementation and its existing tests. Review how parser errors are currently produced, then determine which cases need more descriptive messages and corresponding unit coverage. Done means the parser tests cover the intended error cases and the messages provide useful debugging context.

Written by the indexing model from the issue text.