Finalize BackfillMergeRequestDiffCommitsToPartitioned

What does this MR do and why?

Finalizes the BackfillMergeRequestDiffCommitsToPartitioned BBM.

The two deployment paths require different finalization logic:

• Self-managed: single ensure_batched_background_migration_is_finished call against merge_request_diff_commits.
• GitLab.com: migration was parallelized across four views (merge_request_diff_commits_views_1 through _4), so finalization iterates all BatchedMigration records for the job class via finalize_configurations_for.

Database Review

Changes:

• Adds db/post_migrate/20260722084055_finalise_backfill_commits_to_partitioned.rb

Checklist

• The batched background migration is completed on GitLab.com [*]
• The BBM was added in or before the last required stop (BBM added on %19.0)
• Migration has a corresponding spec
• finalized_by set in the BBM YAML doc
• disable_ddl_transaction! present
• Scoped to gitlab_main_org
• Run db: gitlabcom-database-testing job (link)

[*] Chatops status

Slack link (internal)

!

References

Related to #604753

MR for original migration: !224072 (merged)

MR acceptance checklist

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