command: Add a `filterable` option for externally filtered palettes (#2776)

## Summary

- Add `Command::filterable(bool)` (default `true`): keeps the query
field, spinner and key bindings, but skips the local substring filtering
so every supplied item stays visible.
- For an externally filtered source (e.g. an async server-side search)
the local filter only gets in the way: it drops matches the backend
understood (pinyin, ticker formats), and a query change re-filters the
stale items and hands the highlight to an arbitrary textual match. With
`filterable(false)` a query change resets the highlight to the first
item, and the query still reports through `on_query`.
- Existing palettes are unaffected; `searchable(false)` behavior is
unchanged.

## Test plan

- [ ] `cargo test -p gpui-component command` (33 tests, including the
new `filterable_off_keeps_every_item_and_resets_the_highlight`)
- [ ] Story: the Command examples still filter and highlight as before.

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Co-authored-by: Claude Fable 5 <noreply@anthropic.com>