searchable_list: Store selection by item value

[suxiaoshao]

Fixes #2652

Summary

• store committed searchable-list selections as items and compare identity by Value
• resolve hidden values through item_by_value; SearchableVec searches its complete source
• keep IndexPath only for current visible cursor, click, and change positions
• update Combobox, Select, stories, tests, and bilingual docs

IndexPath identifies a row in the current filtered view, not an option. Filtering can hide an item or make the same path refer to another item, while selection semantics already use Value.

Breaking Changes

• Selection accessors and custom triggers now receive items only.

- &[(IndexPath, D::Item)]
+ &[D::Item]

• Delegate selection hooks now receive item-only selections.

- current_selection: &[(IndexPath, Self::Item)]
+ current_selection: &[Self::Item]

- selection: &mut Vec<(IndexPath, Self::Item)>
+ selection: &mut Vec<Self::Item>

- final_selection: &[(IndexPath, Self::Item)]
+ final_selection: &[Self::Item]

• SearchableListState::remove_selected_index resolves the current visible item and now requires the app context.

- state.remove_selected_index(index)
+ state.remove_selected_index(index, cx)

Custom delegates that need to resolve filtered-out values should override item_by_value.

Validation

• cargo test -p gpui-component test_select_hidden_value_notifies_trigger
• cargo check -p gpui-component --tests
• cargo clippy -p gpui-component --all-targets -- --deny warnings
• rustfmt --edition 2024 --check crates/ui/src/select.rs
• git diff --check

[suxiaoshao]

@codex review

[suxiaoshao]

@codex review