Backport of 'Fix backfill migration overlap on empty CI partitions' to 19-1-stable-ee

What does this MR do and why?

Backport of !254332 (merged).

The post-deploy migration BackfillPipelinesIdRangeOnCiPartitions could abort with PG::ExclusionViolation when a ci_partitions row between the first partition and the current one had no builds. That left a non-current partition with an unbounded range which overlapped the current partition, so the whole db:migrate rolled back and every later migration stayed pending.

The fix skips that write. A partition left without a range falls back to a full scan at lookup time, so pipelines still resolve correctly. As a safety net the backfill now runs without a DDL transaction and tolerates a range overlap on the per-row update; other errors (timeouts, deadlocks) still abort.

The migration shipped in 19.0, so the file exists on every target stable branch and the cherry-pick applied cleanly with no per-branch differences.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, request help in the #s_developer_experience Slack channel to confirm the failures are unrelated to the merge request.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.