Add expectedHeadSha pinning to merge_pull_request (#3182)

* Add head SHA pinning to merge_pull_request

* Update merge_pull_request generated documentation

* test(pull-requests): harden merge SHA pinning

Make the schema and unpinned compatibility coverage explicit, and verify stale-head conflicts surface without a retry.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

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Co-authored-by: Sam Morrow <sammorrowdrums@github.com>
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>