color_picker: add ColorPickerState::set_open and is_open (#2645)

## Summary

`ColorPickerState.open` is private with no setter, so a picker can only
be opened by clicking its own trigger button. Callers that want to drive
it from elsewhere have no supported way to do so.

My use case: a terminal theme editor that renders a preview of coloured
output and lets you click any run of text to edit the colour that
painted it. The click knows which palette slot it needs, but cannot open
that slot's picker. The workaround is to select the slot and make the
user click the swatch a second time.

## Changes

Adds `set_open`/`is_open` to `ColorPickerState`, alongside the existing
`set_value`/`value` pair:

```rust
picker.update(cx, |state, cx| state.set_open(true, cx));
```

`set_open` returns early when the state is unchanged, so it will not
spuriously notify.

17 lines added, no existing behaviour changed, no public API removed or
altered.

## Testing

Typechecked against a real consumer rather than in isolation: pinned my
application to this branch and built it, exercising both
`set_open(true)` and `set_open(false)` from mouse handlers.

I could not get `cargo check` to run on this repo standalone —
`pathfinder_simd` fails with `cannot find function simd_fmin` on my
toolchain (Rust nightly 1.97, macOS aarch64), which reproduces on an
unmodified checkout and so looks unrelated to this change.

## AI disclosure

Per CLAUDE.md's request to mark AI-generated portions: this patch was
written with Claude Code. The diff is small and I have reviewed it.