Add support for ACP context-compaction updates and streamed summaries.
The ACP beta flag gates only the capability advertised to external ACP
agents. Native Zed-agent compaction (manual and automatic), including
the shared rendering, search, and export behavior, remains available
regardless of that flag.

- Apply updates by compaction ID without moving their timeline entries,
preserving omitted/null/replacement semantics for summaries and errors.
- Retain rich summary content and show explicit placeholders for
unsupported content instead of silently dropping it.
- Render progress, completion, failure, cancellation, and unknown
statuses. Expanded summaries and literal error details participate in
thread search, and Markdown exports preserve status and details.
- Keep native compaction compatible: failed retry attempts become
terminal, and cancellation is delivered before the stop event even when
the bridge has not yet applied the queued start event. Preserve native
streaming and save/reload behavior.

Release Notes:

- N/A