Added testing coverage for the SelectionList widget

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

Summary

• Implemented operate() methods for SelectionList and its internal List widget to enable widget tree operations and testability
• Added SelectionList unit tests
• Updated trait bounds to include iced_widget::text::Catalog where needed for text rendering support
• Created integration test file tests/selection_list_integration_tests.rs

# Verify test
cargo test --test selection_list_integration_tests && cargo test --lib selection_list::tests