[AI TestGen] Add tests for ActionButtonsPreference content description

- Add a test to confirm that `setButtonXDescription` correctly sets the content description for each button.
- Add a test to verify that if no content description is set, the button's text is used as a fallback.
- Update the `createMock()` helper method to include mocks for the `setButtonXDescription` methods.

**Caution:** This CL is AI generated based on change: 33749430.

Flag: TEST_ONLY
Change-Id: I3b85b47a1c971a8f07d09dbabdcad55fc27535f4