Add baseline tests for plugin behaviors and queue processing

Problem/Motivation

Right now, the Revision Manager module doesn’t have any automated test coverage.
Before we tag the first release, we should lay down some solid baseline tests to make sure the core functionality works as expected.

This issue will add Kernel tests that cover the most important behaviors:

• Making sure revision cleanup works based on plugin settings (amount, age, etc.).
• Confirming that bundle-level overrides are respected.
• Testing how multilingual revisions are handled.
• Checking that queue batch processing runs smoothly.