Fix safe-output updates for fork pull request branches (#56341)

* Forward PR head baseline to safeoutputs

* test: update safeoutputs compiler expectations

* Improve contributor fork push failure guidance

* Recompile workflows: forward GH_AW_PR_HEAD_REPO to safeoutputs container

Co-authored-by: gh-aw-bot <259018956+gh-aw-bot@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: gh-aw-bot <259018956+gh-aw-bot@users.noreply.github.com>