test(cmux-next): reserve the cloexec probe descriptor

[teamleaderleo]

Summary

The daemon socket CLOEXEC test probed a low descriptor number directly. A shell or unrelated inherited descriptor could occupy that number and make the redirection succeed even when the transport socket was correctly closed on exec.

Duplicate the descriptor into a reserved high range while preserving its descriptor flags before spawning the probe shell. The test still asserts the transport's FD_CLOEXEC bit and retains the no-CLOEXEC control case.

Validation

• python3 scripts/verify-local.py --only swift-syntax --swift-changed origin/feat-cmux-next
• Fleet daemon test job required for native validation

Changelog

Fixed flaky daemon socket close-on-exec coverage.

🤖 Generated with Claude Code

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Summary by cubic

Fixes the daemon socket CLOEXEC test so it no longer falsely passes when an unrelated inherited descriptor occupies the probed number. The test now duplicates the descriptor into a reserved range (≥ 128) while preserving its flags before spawning the probe shell, and still asserts the transport's FD_CLOEXEC bit with the no-CLOEXEC control case intact.

^{Written for commit 7252072. Summary will update on new commits.}

[coderabbitai]

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[github-actions]

Not verified: macOS jobs did not run: compile and app tests were skipped on 7252072794; ci-status is green without them.

CI passes on 7252072794 (run 37506396810 attempt 1).
macOS jobs did not run: compile and app tests were skipped: Fast static checks skipped (the macos job needs both to succeed). Re-run CI once they pass, before merging.

Written by scripts/ci/classify_failures.py (ci-failure-attribution.yml); signatures are its SIGNATURES table. A machine verdict is the runner's fault, not this PR's; yours means the failing file is one this PR changes, also red on main that main's latest full suite fails the same way.

[teamleaderleo]

Exact-head fleet compile completed successfully for 7252072794face1ad38d59c777e46e53ab62c8ea (receipt f2d1e6c12af262718ec5dd90). The descriptor probe remains covered by the PR's focused change; GitHub macOS test lanes are queued behind the same-tree cmux-tui artifact.

[github-actions]

The feat-cmux-next push run https://github.com/manaflow-ai/cmux/actions/runs/37510291185 at 8e8149d failed: cmux-next generated files, cmux-next swift test.
Those jobs last passed at 9872eec. This pull request is one of 1 merged since: #17599.
If the failure is in your change, fix forward on feat-cmux-next. Pull requests run only the tiers their paths reach (docs/ci/cmux-next-tiers.md); label a batch PR full-ci to run them all before merging.

[github-actions]

The feat-cmux-next push run https://github.com/manaflow-ai/cmux/actions/runs/37510568883 at df62960 failed: cmux-next Release compile (Xcode 26), cmux-next swift test.
Those jobs last passed at 9872eec. This pull request is one of 2 merged since: #17599, #17597.
If the failure is in your change, fix forward on feat-cmux-next. Pull requests run only the tiers their paths reach (docs/ci/cmux-next-tiers.md); label a batch PR full-ci to run them all before merging.

[github-actions]

The feat-cmux-next push run https://github.com/manaflow-ai/cmux/actions/runs/37510605955 at 96f7421 failed: cmux-next generated files, cmux-next swift test.
Those jobs last passed at 9872eec. This pull request is one of 3 merged since: #17599, #17597, #17537.
If the failure is in your change, fix forward on feat-cmux-next. Pull requests run only the tiers their paths reach (docs/ci/cmux-next-tiers.md); label a batch PR full-ci to run them all before merging.