ssh: wait for child adoption before releasing the run

[anton-107]

Changes

Add a non-reaping waitid check before releasing the SSH bootstrap run, plus six regression scenarios wired into the SSH Go test suite.

Why

Targets #6387. After SIGTERM, server-group helpers can still parent detached work that the notebook has not adopted yet. An empty survivor scan must not release the run while children remain. The existing SIGCHLD handler retains ownership of exit statuses, and polling/report throttling stay unchanged.

Tests

• Regression cases fail on the unpatched parent (b3c4b6c3c) and pass with this fix. An isolated Linux process reproduction also verifies delayed adoption and exit-status preservation.
• ./task fmt, ./task checks, ./task lint, and ./task test-exp-ssh pass.
• ./task test passes with OMNIGENT unset, cloud testing disabled, and an isolated TMPDIR; two unrelated Terraform acceptance timeouts pass on automatic retry.

This PR was written by Codex.

---

This PR was created with GitHub MCP.

[github-actions]

Waiting for approval

Based on git history, these people are best suited to review:

• @ilia-db -- recent work in experimental/ssh/internal/client/

Eligible reviewers: @andrewnester, @denik, @janniklasrose, @lennartkats-db, @pietern, @rclarey, @renaudhartert-db, @rugpanov, @shreyas-goenka, @simonfaltum

_{Suggestions based on git history. See OWNERS for ownership rules.}

[eng-dev-ecosystem-bot]

Integration test report

Commit: 136a9e0

Run: 35278488532

	Env	✅​pass	🙈​skip	Time
✅​	aws linux	276	19	4:10
✅​	aws windows	278	17	4:05
✅​	azure linux	275	19	4:05
✅​	azure windows	277	17	3:41
✅​	gcp linux	276	19	4:16
✅​	gcp windows	278	17	3:53

Top 3 slowest tests (at least 2 minutes):

duration	env	testname
4:03	aws windows	TestAccept
3:50	gcp windows	TestAccept
3:39	azure windows	TestAccept