cancelBackgroundCompaction() emits no compaction_complete, so "Compacting conversation…" stays on screen #5394

Describe the bug

After an SDK extension cancels a background compaction with session.rpc.history.cancelBackgroundCompaction(), the compaction stops but the session never emits session.compaction_complete. The app's "Compacting conversation…" line therefore stays in the chat — still there 7.5 minutes later, mid-turn — while the session carries on normally. Users read it as a compaction in progress, or a stuck session.

The call also returns { cancelled: false } when it does cancel (3 of 3 in a probe, then across three end-to-end runs), so the caller cannot tell success from failure either.

Frequency: every cancellation.

Extensions: this is the extension API's own behaviour; reproducing needs an extension that calls it.

Wanted labels: area:sessions, area:context-memory, type: Bug.

Affected version

Runtime 1.0.102-0.unstable.r41779900948.gf482387 (desktop app 1.1.35).

Steps to reproduce

1. In an extension, listen for session.compaction_start with trigger: "threshold".
2. Call session.rpc.history.cancelBackgroundCompaction() within ~150 ms.
3. Observe: the call resolves { cancelled: false }; no compaction runs (no summary, tokens unchanged); no session.compaction_complete is ever written; the app keeps showing "Compacting conversation…".

Expected behavior

A cancelled compaction emits session.compaction_complete with outcome: "cancelled" — the event schema already has that value, and this machine's logs hold two such completions from other cancellation paths — and the call returns cancelled: true.

Additional context

App version         1.1.35
OS                  Windows 10 Enterprise 26H2 (build 26300.9711)
Architecture        AMD64
Copilot CLI         copilot-sdk-runtime-1.0.102-0.unstable.r41779900948.gf482387