combobox: Add value-based selection API (#2576)

Closes #2575

## Description

Adds `ComboboxState::set_selected_values` to resolve values through the
current delegate and replace the committed selection without emitting
user-originated events. This keeps programmatic selection aligned after
`set_items`, synchronizes the adapter snapshot, ignores unknown values,
and preserves caller order.

Includes focused tests for current delegate resolution, missing and
empty values, snapshot synchronization, and event behavior, plus English
and Simplified Chinese documentation.

## How to Test

- `cargo check -p gpui-component --tests`
- `cargo clippy -p gpui-component --all-targets -- --deny warnings`
- `rustfmt --edition 2024 --check crates/ui/src/combobox.rs`
- `git diff --check`

## Checklist

- [x] Reviewed AI-assisted changes for correctness.
- [N/A] Story testing; no visual behavior changed.
- [N/A] Platform testing; the API is platform-independent.