Stop reuse self-tests posting fake CI notices

[Mikola Lysenko (mikolalysenko)]

Problem

Every lint-ecosystems run (PR, merge queue and main push) posts 10 fake notice annotations, for example on main's latest push run (check run 117423824414 at 7d3086f):

Could not verify merge-queue CI (TimeoutExpired); running all checks.
Could not compare with the previous PR head (CalledProcessError); running all checks.
… 10 in total

Nothing failed. The reuse scripts never run in that job. These lines come from the job's python3 -B -m unittest discover -s scripts/tests step: the fallback self-tests let ci-reuse-merge-group.py and ci-reuse-pr-head.py print their real ::notice:: workflow commands to stdout, and GitHub turns those lines into annotations. A local run of the suite prints 15 of them, and GitHub keeps 10 (its per-step cap).

The harm:

• A reader can take the annotations to mean the merge-queue reuse check or the PR-head reuse check failed, which sends anyone debugging a CI-reuse decision down the wrong path.
• They fill the step's notice cap, so a real notice from that step would be dropped.

CI is otherwise healthy this run: the merge queue is empty, nothing is queued or running, main is green, and the only failed run in 3 days (38806080761) was fixed by that PR's next push. So this falls under priority 4 (make CI more robust).

Root cause

test_api_failure_timeout_and_bad_payload_* in scripts/tests/test_ci_reuse_merge_group.py and scripts/tests/test_ci_reuse_pr_head.py call reusable_run on its error path without capturing stdout.

Fix

A small assert_falls_back helper in each test file. It runs reusable_run under contextlib.redirect_stdout, asserts that the result is None (unchanged), and also asserts that the ::notice:: fallback line was printed. The fallback is now checked, not just silenced. No production script or workflow changes.

Proof

• python3 -B -m unittest discover -s scripts/tests: 347 tests, OK (6 skipped). Lines starting with :: in stdout went from 15 to 0.
• Negative check: removing ::notice:: from ci-reuse-merge-group.py makes the new assertion fail 7 subtests.

Removed or moved tests

None. Every test still runs in the same place (lint-ecosystems).

https://claude.ai/code/session_01SeHhBxEvePf8ERTaTTenMK

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[Mikola Lysenko (mikolalysenko)]

bugbot run

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[Mikola Lysenko (mikolalysenko)]

Labeled Ready for review (burn-down agent).

• Head: 564c31a
• CI: 54/54 green (35 success, 19 skipped). No main-wide failures.
• Bugbot: reviewed 564c31a, no findings. 0 open threads.
• Mergeable with no conflicts. CHANGELOG.md is not touched.
• Reviewer focus: only the two reuse self-tests under scripts/tests/ change; check they no longer emit real ::notice annotations.

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