Swap merge_request_diff_files with partitioned table

What does this MR do and why?

Completes Step 4 of int-range partitioning for merge_request_diff_files by swapping the partitioned table into place as the primary table.

This is the final step in the partitioning migration:

• Step 1 (18.3): Create partitioned copy with sync trigger
• Step 2 (18.3): Enqueue backfill migration
• Step 3 (18.10): Finalize backfill after required stop
• Step 4 (19.2): Swap tables ← this MR

Dependencies

✅ Dependency merged: !247724 (merged) has been merged.

The partitioned table has a NOT NULL constraint on project_id. During project import, MergeRequestDiffFile objects are bulk inserted without project_id being set, which causes import failures. !247724 (merged) fixed this by propagating project_id from the parent MergeRequestDiff before bulk insert.

Changes

1. FK validation migration - Validates foreign keys on the partitioned table. No-op on GitLab.com (already validated), ensures self-managed instances have validated FKs before swap.

2. Swap migration - Uses replace_with_partitioned_table helper to:

   • Rename merge_request_diff_files → merge_request_diff_files_archived
   • Rename merge_request_diff_files_99208b8fac → merge_request_diff_files
   • Create sync trigger pointing to archived table (for rollback support)

3. Model update - Adds PartitionedTable concern and partitioned_by declaration

4. Partition manager - Updates registration to use new table name

5. Spec cleanup - Removes obsolete sync trigger specs that tested the intermediate state

Partitioning details

• Strategy: int_range by merge_request_diff_id
• Partition size: 200M rows per partition
• Sharding key: project_id

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related issues

Closes #422771 (closed)

Database

Migration: ValidateFksOnMergeRequestDiffFiles99208b8fac

No-op on GitLab.com (FKs already validated). For self-managed, validates:

• fk_rails_ebcce501f5 (project_id → projects)
• fk_rails_6fff895059 (merge_request_diff_id → merge_request_diffs)

Migration: SwapMergeRequestDiffFilesWithPartitionedTable

Uses replace_with_partitioned_table helper which:

1. Drops sync trigger on original table
2. Renames original → archived
3. Renames partitioned → original name
4. Creates new sync trigger pointing to archived table