Avoid commit preload timeout in MR destroy audit

Eugenia Grieffrequested to merge
602304-fix-mr-destroy-auditor-times-out into master

What does this MR do and why?

What does this MR do and why?

Deleting a merged MR with many commits can cause a timeout in MergeRequestBeforeDestroyAuditor. The audit event was fetching committer IDs by loading all commits via MergeRequestDiff#with_users, which triggers a full association preload on merge_request_diff_commits.

Adds committer_user_ids_from_diff on MergeRequest, which queries the commits tables directly (reusing the existing committer_emails_from_diff SQL) and resolves user IDs via User.by_any_email. This avoids loading commit objects entirely.

Gated behind FF destroy_mr_audit_use_committer_ids_from_diff since committer_emails_from_diff has limited production exposure so far.

References

Related to #602304

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