Replace call sites that treated the active thread as the root thread
with the dedicated `root_thread_view` and `root_thread` helpers, and
simplify `ConversationView` lookups to avoid passing `cx` when it is
not needed.

Also emit token limit telemetry when usage crosses warning or exceeded
thresholds, while avoiding duplicate events for the same session state.

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 is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A

Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>