CI: fix milestone-tag-assistant race when labels change post-merge

[potiuk]

The `milestone-tag-assistant.yml` workflow snapshots PR labels at the `get-pr-info` job (via `listPullRequestsAssociatedWithCommit`) and then spends ~1.5 minutes installing Breeze and running `breeze ci set-milestone`. If a maintainer adds and removes a backport label inside that window, the action commits to the stale-snapshot decision and sets the wrong milestone.

This happened on #67301: a `backport-to-v3-2-test` label that lived for 49 seconds caused an `Airflow 3.2.3` milestone to be set on a `main`-only documentation PR. The workflow's comment justifying the decision was posted ~1.5 minutes after the label had already been removed.

Timeline (PR #67301)

Time	Actor	Event
20:31:18	shahar1	Merges PR (no milestone, no backport label)
20:31:28	shahar1	Adds `backport-to-v3-2-test` (+10 s)
20:32:17	shahar1	Removes `backport-to-v3-2-test` (+49 s after adding)
20:33:49	github-actions[bot]	Sets milestone `Airflow 3.2.3` ❌
20:33:51	github-actions[bot]	Posts justifying comment
next morning	eladkal	Removes the incorrect milestone

Fix

Re-read `issue.labels` from the freshly-fetched issue (which is already in scope after the existing `milestone is not None` check) before computing the milestone. If the labels changed since the snapshot:

• Honour any skip label that appeared after the snapshot.
• Re-run `_determine_milestone_version` with the current labels and use the fresh decision; if the decision flips to "no milestone", bail out before posting the comment.

Adds three regression tests covering the three race-window cases (backport label removed, replaced, skip label added) and updates two existing happy-path tests to populate `mock_issue.labels` so the re-read sees the same labels as the snapshot.

---

Was generative AI tooling used to co-author this PR?

• Yes — Claude Code (Opus 4.7)

Generated-by: Claude Code (Opus 4.7) following the guidelines

[jason810496]

Thanks for catching the race condition! (I didn't foresee this situation when developing)

[jason810496]

If I understand the event correctly, we should add additional check in _should_skip_milestone_tagging.
If there's any event that maintainer remove any backport label themself. We need to skip the auto tagging.

Even with the new if set(current_labels) != set(labels): check, I don't think the existing implementation of _should_skip_milestone_tagging would be enough.

airflow/dev/breeze/src/airflow_breeze/global_constants.py

Lines 832 to 833 in d0f981c

	# Labels that indicate the PR should be skipped from milestone auto-tagging
	MILESTONE_SKIP_LABELS: frozenset[str] = frozenset({"area:dev-tools", "area:dev-env", "area:CI"})

airflow/dev/breeze/src/airflow_breeze/commands/ci_commands.py

Lines 1142 to 1144 in d0f981c

	def _should_skip_milestone_tagging(labels: list[str]) -> bool:
	"""Check if the PR should be skipped from milestone auto-tagging."""
	return bool(set(labels) & MILESTONE_SKIP_LABELS)

Btw, I can help follow-up for this issue if you don't have bandwidth at this moment.