fix: cherry-pick DCO check fails due to author email mismatch from squash merge

[rnetser]

Problem

When the webhook server cherry-picks a squash-merged commit, the DCO check on the cherry-pick PR fails because:

1. GitHub squash-merge rewrites the commit author email to the noreply format (e.g., 86722603+user@users.noreply.github.com)
2. The Signed-off-by trailer preserves the original email (e.g., user@redhat.com)
3. git cherry-pick replays the commit with the mismatched emails
4. The DCO app checks that Signed-off-by email matches the commit author email → fails

This requires manual sign-off approval for every cherry-picked PR.

Root Cause

GitHub's squash-merge changes the commit author identity from the real email to the GitHub noreply format, but preserves the original Signed-off-by trailer verbatim. The cherry-pick faithfully reproduces this mismatch.

Solution

Add _restore_original_author_for_cherry_pick() to runner_handler.py that:

• Extracts the Signed-off-by trailer from the cherry-picked commit message
• Compares the sign-off email against the current commit author email
• If they differ, amends the commit author to match the sign-off (restoring the original PR owner's identity)
• Called after successful cherry-pick (clean or AI-resolved) before push

Deliverables

• Add _restore_original_author_for_cherry_pick() method to RunnerHandler
• Call it in cherry_pick() before push
• Add unit tests (7 tests covering all paths)
• Pass all reviewers (quality, guidelines, security, Cursor peer review)
• All tests pass (1538 tests, 90.43% coverage)

References

• Original PR that was signed off: [Storage] Update owners RedHatQE/openshift-virtualization-tests#4461
• Cherry-pick PR that needed manual sign-off: CherryPicked: [cnv-4.20] [Storage] Update owners RedHatQE/openshift-virtualization-tests#4470