fix(chat): deliver background job notifications at step boundaries

[zhanba]

Summary

• Background job completion notifications (from executeCommand with background: true) were only handed to the model once the whole turn had ended. Mid-loop they sat in the queue, so they arrived late and often after the model had already moved past the point where they were actionable.
• Deliver them at the agent loop step boundary instead, riding along with the continuation request that is issued anyway — no extra request, latency is now at most one step.
• packages/vscode-webui: the auto-dequeue effect in chat-toolbar.tsx gated delivery on taskStatus being idle. Delivery is now also attempted from the auto-continuation decision chain (page.tsx -> continueAutomatically), so it inherits every existing safety gate: an intentional pause (autoApproveGuard === "stop", paused todo loop) is never resumed by a notification, and sendQueuedMessage(..., { keepAutoApproveGuard: true }) makes sure a one-shot manual approval is not upgraded to auto. Only a background-job notification sitting at the queue head is eligible; user-typed queued messages keep their previous behaviour. Dedup is keyed on message identity (deliveredNotificationsRef) rather than an in-flight boolean, so an aborted send cannot stall the queue.
• packages/cli: TaskRunner had the same gate — it only drained pendingBackgroundJobNotifications on a "finished" step. The drain now also happens right before the continuation chat.sendMessage() when the step result is "next". The decision lives in a new pure helper, src/lib/background-job-notification-delivery.ts, so it is unit-testable without the full TaskRunner harness. "retry" steps never inject (the retry must re-send the same state), and the end-of-task waitForAsyncWork() drain is untouched.

Not in scope: packages/cli/src/running-task-adaptor.ts creates per-task BackgroundJobManagers but never subscribes to onDidFinish, so that path surfaces no background job notifications at all. Pre-existing, worth a follow-up.

Test plan

• cd packages/cli && bun run test — 16 files / 132 tests pass, including 6 new tests for the delivery helper (drains all pending events into one message; "next" delivers and empties the queue; "retry" / "finished" / empty queue do not).
• cd packages/vscode-webui && bun run test — 77 files / 532 tests pass, including new coverage for the queue helpers, sendQueuedMessage (guard preservation, single-entry removal) and the toolbar delivery callback (head notification delivers once; user message ahead blocks it).
• bun tsc clean in both packages; bun fix / bun check clean at the repo root.
• Manual: start a 25s background job, then run a sequence of short tool calls — the notification now arrives mid-loop instead of after attemptCompletion; two jobs finishing near the same boundary are batched into one notification message; a job started immediately before attemptCompletion is still awaited and delivered by the existing end-of-task drain.

Note: the local pre-push hook could not run packages/vscode's extension tests in this worktree (IPC handle ... is longer than 103 chars — the checkout path exceeds the macOS unix socket limit). Every other hook step passed; this change does not touch packages/vscode.

🤖 Generated with Pochi | Task

[codecov-commenter]

Codecov Report

❌ Patch coverage is 74.07407% with 28 lines in your changes missing coverage. Please review.
✅ Project coverage is 33.38%. Comparing base (ab3dc4d) to head (1db7386).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
packages/vscode-webui/src/features/chat/page.tsx	0.00%	19 Missing ⚠️
packages/cli/src/task-runner.ts	0.00%	9 Missing ⚠️

❌ Your patch status has failed because the patch coverage (74.07%) is below the target coverage (75.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1932      +/-   ##
==========================================
+ Coverage   33.32%   33.38%   +0.06%     
==========================================
  Files         778      779       +1     
  Lines       83120    83210      +90     
  Branches     4897     4757     -140     
==========================================
+ Hits        27696    27780      +84     
- Misses      55304    55310       +6     
  Partials      120      120              

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