Swap bigint columns on merge_requests stage three

What does this MR do and why?

This is the final MR where we swap stage 3 bigint columns for merge_requests table.

In before MRs, we have prepared indexes (async and sync), and FKs (async validation). And this MR, we perform below things,

• Add sync FK validation for non-GitLab.com ENVs
• Add final step for partition FK validation
• Swap stage 2 bigint columns (id, author_id) and related indexes and FKs
• Drop temp indexes, FKs after swap (leave columns for final removal together after one milestone stop)

References

• Issue - Swap MergeRequest ID columns (Step 2) (#507695 - closed)
• Previous MR for swapping bigint columns - Swap bigint columns on merge_requests stage two (!216433 - merged)
• Previous MR for adding FKs - Prepare foreign keys for bigint conversion on m... (!221813 - merged)

Screenshots or screen recordings

NA

How to set up and validate locally

NA

Process Plan

This MR would follow unstandard process, and would not run on GitLab.com via normal PDM.

• Merge Swap bigint columns on merge_requests stage three (!225419 - merged)
• Confirm Swap bigint columns on merge_requests stage three (!225419 - merged) is deployed to prod
• Skip the migrations 20260310071802, 20260310071803 and 20260310071804 via ChatOps only for main
• Let the PDM job execute them for ci and sec
• Execute migrations for main manually via CR

MR acceptance checklist

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