ci: give each Python lane test an empty Foundation home

[teamleaderleo]

Summary

In main run 36398180780, app-host shard 4 failed 11 Python lane tests on one owned runner. Among them were test_cli_socket_operation_deadline, test_cli_layout_focus_contract, test_browser_profile_cli and the test_cli_claude_teams_* tests. Their fake sockets received a line that was not JSON, or closed before replying.

The cmux CLI reads its socket password file under Foundation's home directory, and that directory follows CFFIXED_USER_HOME, not HOME. When the runner account also runs its own cmux with a socket password, the CLI sends auth <password> to every fake socket before the request under test. #15204 fixed the same failure in the Swift CMUXCLIEventsStreamErrorTests on the same runner.

run_python_test_lane.py now points CFFIXED_USER_HOME at an empty per-test directory, unless the job or the test already sets it. Tests that set their own isolated home are unchanged.

Testing

• Called run_one on a stub test locally: it receives a fresh per-test CFFIXED_USER_HOME.
• python3 -m py_compile scripts/ci/run_python_test_lane.py.

Changelog

none

🤖 Generated with Claude Code

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Summary by cubic

Fixes flaky Python lane tests on runners whose account also runs its own cmux.

• The CLI reads its socket password under CFFIXED_USER_HOME (not HOME), sending an auth line to every fake socket fixture before the request under test, which broke 11 tests in one main run.
• run_python_test_lane.py now points CFFIXED_USER_HOME at an empty per-test directory unless the job already sets it; tests may still set their own isolated home for subprocesses.

^{Written for commit f6c7057. Summary will update on new commits.}

[github-actions]

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📥 Commits

Reviewing files that changed from the base of the PR and between b36339a and f6c7057.

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• scripts/ci/run_python_test_lane.py

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[github-actions]

Dogfood build of f6c705731f57616646b6603140f7ccb1da37ab34

cmux DEV pr-15289-f6c70573.app

The link opens this exact commit in the cmux dev menu bar app. The build starts on each push and the page waits until it is ready; a newer push replaces it. It signs in against production, so Cloud or backend changes still need a tagged build with a development backend.

[github-actions]

Merge receipt for f6c705731f: every check was green at merge (20 verified; 21 skipped by policy). Full suite runs on main after merge.