combobox: expose search query accessors (#2663)

## Summary

- add `ComboboxState::query()` to read the current search query
- add `ComboboxState::set_query()` to update the query and immediately
refresh filtered items
- update the Combobox footer example so the typed query can be added as
a new university
- add regression coverage for both the query text and filtered results

## Testing

- `cargo test -p gpui-component
test_combo_box_set_query_updates_text_and_filters_items --lib`
- `cargo test -p gpui-component combobox --lib`
- `cargo test -p gpui-component --lib`
- `cargo check -p gpui-component-story`

## Manual verification

1. Run the Combobox story.
2. Open the **Footer** example.
3. Type a university name.
4. Click **New university** and confirm the typed query is added to the
options.

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Co-authored-by: Jason Lee <huacnlee@gmail.com>
Co-authored-by: Codex <codex@openai.com>