Add Unit Tests

[ashleyshaw]

Description
Introduce a suite of PHP unit tests, focusing first on importer functionality.

Why This Matters

• Automated testing reduces bugs and ensures reliability.
• Importers are complex and benefit significantly from robust testing.

Tasks

1. Set up a testing framework (e.g., PHPUnit, WP test suite).
2. Add test cases for importer code paths and edge cases.
3. Integrate tests into the plugin’s CI pipeline.