test: scratch probe for update-branch 422 variants

[eleshar]

Scratch branch used only to observe the exact GitHub API responses for PUT /repos/{owner}/{repo}/pulls/{n}/update-branch so the classifier in #3693 can be written against real message text rather than a guess. Not for merge. Will be closed and deleted.

Deliberately behind develop by one commit and mergeable, so the endpoint reaches the head-SHA check rather than short-circuiting on staleness.

Labels: type:ci, area:ci, priority:minor, status:needs-review.

Linked issues

Relates to #3693, #3574

Checklist (Global DoD / PR)

• All AC met and demonstrated — n/a, test branch
• Tests added/updated — n/a, test branch
• Code/design reviews approved — do not review
• CI green — n/a, test branch

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[eleshar]

Scratch probe for #3693, closing and deleting as planned. Purpose and observations are recorded on #3693.