Zed did not advertise document highlight dynamic registration support or
handle `textDocument/documentHighlight` registrations. Dynamically
registered providers therefore fell through to the unhandled-capability
path, while servers with a fallback had to advertise a static provider
for every attached document.

Advertise dynamic registration support and track document highlight
registrations and unregistrations alongside the other selector-aware
dynamic text document capabilities. Refresh editors whenever the
effective document highlight registration state changes, including
selector-only changes that leave the merged provider unchanged, so
runtime registrations take effect immediately and unregistrations clear
stale or in-flight highlights.

Add coverage for the client capability, registration lifecycle with
multiple selectors sharing identical provider options, and editor
refresh behavior when an applicable selector is added or removed.

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed’s UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A

---------

Co-authored-by: GPT-5.6 Sol <noreply@openai.com>
Co-authored-by: Kirill Bulatov <kirill@zed.dev>