fix: defer early session.idle instead of dropping it (#1517)

[code-yeongyu]

Summary

Fixes #1517 — Background tasks launched via delegate_task(run_in_background=true) permanently hang when the agent responds in under 5 seconds.

Root Cause

The session.idle handler in BackgroundManager has a guard that rejects idle events that fire within MIN_IDLE_TIME_MS (5s) of task start. Since session.idle is edge-triggered (fires once, not periodically), dropping the early event means it never fires again → task stuck forever.

Fix

Instead of dropping the early idle event, defer it using setTimeout(remainingMs). When the deferred timer fires, it re-checks whether the task is still running before proceeding with completion validation. This preserves the original intent (don't complete prematurely) while not losing the idle signal.

Changes

• src/features/background-agent/manager.ts: Replace early idle return with setTimeout deferral + cleanup logic
• src/features/background-agent/manager.test.ts: Add TDD tests for early idle deferral behavior

Testing

• ✅ New tests for early idle deferral (3 cases)
• ✅ All existing tests pass (no regression)
• ✅ bun run typecheck passes
• ✅ bun run build passes

---

Summary by cubic

Prevents background tasks from hanging when session.idle fires within 5s by deferring the event instead of dropping it. Keeps the minimum idle window while ensuring the idle signal is not lost.

• Bug Fixes
  • Defer early session.idle with a timer, then re-check task state and re-handle the event.
  • Track and clear deferred idle timers on completion, abort/remove, and shutdown to avoid leaks.
  • Add tests for early deferral, after-threshold handling, and no-op when the task completes before the deferred idle fires.

^{Written for commit 796bf79. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.