[ML] Skip backport for minor-freeze version bump PRs

[edsavage]

Summary

After the 9.5 minor freeze, merging #3068 (Leg B main bump to 9.6.0) triggered the automatic backport workflow and opened spurious #3069 onto branch 9.5. That PR would have incorrectly bumped the release branch to 9.6.0; it has been closed without merging.

This happens because:

• elasticsearchmachine labels minor-freeze PRs with the release version (e.g. v9.5.0)
• The same merge adds the new release branch to targetBranchChoices in .backportrc.json
• The generic branchLabelMapping rule then backports the main-only freeze commit onto the new release branch

This will recur on every minor freeze unless we skip backport for these automation PRs.

Changes

• Add a no-backport label to minor-freeze PRs opened by bump_main_minor_freeze.sh
• Skip the Backport workflow when that label is present, and as a fallback when the PR head branch is a minor-freeze topic branch (ci/ml-cpp-minor-freeze-main-*)
• Add unit tests guarding both the workflow and the bump script

The no-backport label already exists on the repo.

Test plan

• python3 -m pytest dev-tools/unittest/test_backport_workflow.py -v
• CI dev-tools pytest step passes
• After merge: dummy PR backport-skip smoke test (see Manual verification below)
• Next production minor freeze: Leg B PR has no-backport and does not spawn a spurious backport PR

Manual verification

After this PR merges to main — backport skip smoke test

No need to wait for the next minor freeze. Once #3070 is on main:

1. Open a harmless test PR to main (e.g. a trivial dev-tools comment) with:
   • Labels: no-backport, a version label (e.g. v9.6.0), :ml, >test
2. Merge that PR.
3. Confirm:
   • Actions → Backport for that merge: the Backport PR job is Skipped (workflow if is false).
   • No new PR with the backport label referencing the test PR.

This reproduces the #3068 → #3069 failure mode (version label on a main-only change) without running the version-bump pipeline. Revert or close the test PR afterward.

On the next production minor freeze

When release-eng triggers ml-cpp-version-bump with WORKFLOW=minor again:

• Leg B PR opened by bump_main_minor_freeze.sh should include the no-backport label and be created from a ci/ml-cpp-minor-freeze-main-* branch.
• Merging that PR should not open a spurious backport onto the new release branch.

Note: sandbox refs (testing-*) skip Leg B in CI, so they cannot be used for a full Leg B label E2E test.

Review notes

Addressed Copilot feedback:

• Replaced the broad (minor freeze) title-substring fallback with a head-ref check on the automation topic branch (ci/ml-cpp-minor-freeze-main-*) so unrelated PRs mentioning a minor freeze in their title are not skipped.
• Made the guard-rail tests tolerant of whitespace/quote-style changes in backport.yml via regex.

[prodsecmachine]

✅ Snyk checks have passed. No issues have been found so far.

Status	Scan Engine	Critical	High	Medium	Low	Total (0)
✅	Open Source Security	0	0	0	0	0 issues
✅	Licenses	0	0	0	0	0 issues

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[elasticsearchmachine]

Pinging @elastic/ml-core (Team:ML)

[Copilot]

Pull request overview

This PR adjusts the repository’s automated backporting behavior to avoid creating incorrect backport PRs when the minor-freeze version bump on main is merged (a commit that should remain main-only but may carry release-version labels).

Changes:

• Add a no-backport label to the PR created by dev-tools/bump_main_minor_freeze.sh.
• Update the Backport GitHub Actions workflow to skip when no-backport is present (and also when the title contains (minor freeze)).
• Add unit tests to guard the workflow skip conditions and the bump script’s label application.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
dev-tools/unittest/test_backport_workflow.py	Adds tests that assert the workflow and bump script contain the backport-skip mechanisms.
dev-tools/bump_main_minor_freeze.sh	Ensures minor-freeze PRs get a no-backport label when created.
.github/workflows/backport.yml	Skips running the backport job for PRs labeled no-backport (and those with (minor freeze) in the title).

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