Testing coverage for the DropDown widget

#385 in iced-rs/iced_aw — merged 2025-12-29

Added testing coverage for the DropDown widget

Changes:

• Added operate() method to DropDown overlay implementation to expose children for testing
• Added unit tests
• Created integration test

# Unit tests
cargo test widget::drop_down

# Integration tests
cargo test --test drop_down_integration_tests