feat(tests): increase code coverage with new unit and instrumented tests

[Nacchofer31]

Added 8 new test files, raising line coverage to 97.7% (well above the 70% minimum threshold configured in JaCoCo).

Unit tests (commonTest)

• RandomFilmStateTest — verifies default values and copy operations on the ViewModel state.

Instrumented tests (androidInstrumentedTest)

• FilmHeaderTest — logo display, info button visibility and click callback.
• FilmSearchModeSwitchTest — toggle between INTERSECTION and UNION modes.
• LoadingOrPromptTest — loading indicator and associated texts.
• UserNameTagTest — click, long click and remove icon on user tags.
• FilmErrorViewTest — titles and subtitles for each error type (NO_RESULTS, NO_INTERNET, UNKNOWN, SERVER).
• RandomFilmInfoViewTest — roulette icon, main text and usage tips.
• UserNameRepositoryImplTest — integration tests using an in-memory Room database, covering add (no duplicates), delete and query logic.