fix(core): await child background work before subagent completion

[Ploppy3]

Issue for this PR

Closes #48826

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

A subagent that ends a turn while it still has pending background work (shell or nested subagent with background: true) is reported to its parent as completed at that first idle. The subagent job settled as soon as sessions.resume(child) returned, so the parent received the child's early reply; when the pending work later woke the child and it produced its real result, nothing delivered it.

• SubagentCompletion.finalText runs the child to quiescence before reading its response: while the child still owns pending background notifications that will wake it, it waits for their delivery and resumes again.
• New Job.awaitBackground(notificationID) resolves when the notification is admitted (its durable marker clears). Waiting for job settlement alone races that asynchronous delivery.
• Live subagent jobs and restart recovery share the same body, so recovered jobs behave identically.

How did you verify your code works?

cd packages/core && bun test test/job.test.ts test/tool-subagent.test.ts — the new regression test (waits for pending child background work before notifying the parent) fails when run against the un-fixed source (Expected "running", Received "completed") and passes with the fix.

Reproduced the original failure on v2: the parent was notified with SHELL_STARTED while the child's background shell was still running, and the child's later WOKEN_RESULT_COLLECTED stayed stranded in the child session.

Screenshots / recordings

Not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[Ploppy3]

The description now follows the PR template section for section and the compliance check passes:

• ### Issue for this PR — Closes #48826
• ### Type of change — bug fix checked
• ### What does this PR do?
• ### How did you verify your code works?
• ### Screenshots / recordings — not a UI change
• ### Checklist — both items checked

One note for maintainers on the remaining needs:issue label: it can't clear itself for PRs targeting v2. The pull_request_target run uses the workflow copy without the v2 body fallback, so it only reads closingIssuesReferences, and GitHub returns 0 closing references for PRs that don't target the default branch. Other v2 PRs with a Closes reference (e.g. #48928, #49295) carry the same label. The issue link is present in the description.