fix(coding-agent): suppress aborted compaction stream errors

[code-yeongyu]

Summary

• Treat a summary stream rejection as expected cancellation when the compaction caller signal is already aborted.
• Preserve ordinary provider and stream errors unchanged.
• Add focused coverage for the late stream-result rejection reported after pressing ESC.
• Document the fork behavior and merge-conflict surface.

Root cause

The compaction watchdog can stop consuming as soon as ESC aborts the caller signal, while the provider stream's final
result rejects a moment later with Assistant message stream consumption was cancelled. That late rejection escaped
runExtensionCompaction() and was reported by the builtin extension runner after the normal cancellation notice.

Verification

• RED: focused cancellation case rejected with the exact reported error while the ordinary-error case passed.
• GREEN: test/compaction/speculative-abort-cancellation.test.ts — 2/2 passed.
• Compaction suite: 47 files, 327 tests passed.
• npm run check: passed.
• Latest origin/main merged; post-merge focused tests, compaction suite, and npm run check passed.

Manual QA

Source Senpi TUI in a persistent PTY:

1. Seeded a clean exact session with two large turns.
2. Forced pre-prompt compaction with isolated 16K model metadata.
3. Held an OpenAI-compatible SSE summary stream after its first delta.
4. Injected ESC only after the server confirmed stream consumption.

Observed:

• Context overflow detected, compacting... (esc to cancel)
• Compacting... (esc to cancel) partial held summary
• Auto-compaction cancelled
• No Extension "<builtin:compaction>" error
• No Assistant message stream consumption was cancelled stack
• Editor accepted SESSION-USABLE-AFTER-CANCEL

Evidence is stored locally under local-ignore/qa-evidence/20260803-compaction-esc/, including an xterm.js-rendered PNG.

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Summary by cubic

Suppress spurious compaction errors after ESC by treating late summary stream rejections as normal cancellation when the caller AbortSignal is aborted. Real provider/stream errors still bubble up; adds a focused regression test.

• Bug Fixes
  • In runExtensionCompaction(), return undefined if generateSummaryMessage rejects and the caller signal is already aborted; otherwise rethrow.
  • Stops the late "Assistant message stream consumption was cancelled" error from surfacing after "Auto-compaction cancelled".
  • Adds speculative-abort-cancellation.test.ts to verify both the aborted and ordinary failure paths.

^{Written for commit 1116bd8. Summary will update on new commits.}