Added tests for the MenuBar widget

#387 in iced-rs/iced_aw — merged 2025-12-30

Menu Bar Testing Summary

1. Verified Existing operate() methods in widget and overlay

2. Added unit tests to src/widget/menu/menu_bar

3. Created tests/menu_bar_integration_tests.rs

  cargo test --lib menu_bar::tests --features menu && cargo test --test menu_bar_integration_tests --features menu