cmux-next CI: run OmnibarFieldEditorTests in the isolated field-editor step

[azooz2003-bit]

Summary

OmnibarFieldEditorTests.markedTextNeverCompletesAndItsCommitDoes fails intermittently on CI (for example #17520, job 112399297991: isComposing false, 35.8 s) and blocks unrelated PRs.

The failure isn't a check that runs too early. On a fleet mini, running the whole package the way CI does, the failure reproduced in 1 of 4 runs. In that run, the field editor itself dropped its IME marked text partway through the test (editor.hasMarkedText() false while it was still first responder), and the omnibar state followed. No wait inside the test can fix that. With the suite alone (8 runs) or with only CmuxNextBrowserTests at CI's width (3 runs), it passed every time.

So the suite now runs in the existing isolated field-editor step, next to OmnibarTypingTests, which runs alone for the same reason ("unrelated UI suites can change focus"). It's skipped in the shared step, and every test still runs.

run	suite passed
whole package, width 6	3 of 4
suite alone	8 of 8
CmuxNextBrowserTests only, width 6	3 of 3

Verification

• The repro probe ran on the fleet at the base commit, with print-only diagnostics. The probe branch has been deleted.
• This PR edits only the workflow. The cmux-next swift test job on this PR runs the suite in the isolated step.
• This lane runs without sub-agents, so no review subagent was used.

Changelog

none

🤖 Generated with Claude Code

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Summary by cubic

Moves OmnibarFieldEditorTests into the existing isolated field-editor CI step so its intermittent IME-related failures no longer block unrelated PRs.

• Fails 1 of 4 full-package runs because the field editor drops its IME marked text mid-test; no wait inside the test can fix that.
• Passed 8 of 8 runs alone and 3 of 3 with only CmuxNextBrowserTests.
• Skips the suite in the shared package-test step; all tests still run.
• Touches only .github/workflows/cmux-next.yml.

^{Written for commit 787ab79. Summary will update on new commits.}

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