Backport of 'Fix ci_finished_builds engine swap on deferred post-deploy' into 19.0

What does this MR do and why?

Backports !240219 (merged), !240627 (merged), and !240798 (merged) (merged commit b4f205d5) to 19-0-stable-ee.

ClickHouse post-deployment migration 20260108073937_alter_ci_finished_builds_engine_with_version failed in two ways on stable branches:

1. Structure mismatch (#593129 (closed)): Code: 122. DB::Exception: Tables have different structure. (INCOMPATIBLE_COLUMNS) when self-managed customers upgrade from 18.8.x to 18.9.x–19.0.x. Root cause: hardcoded CREATE TABLE for the tmp table; later-timestamped migrations add columns/projections before this post-deploy migration runs. Fixed in master by !240219 (merged) (use CREATE TABLE ... AS source ENGINE = ... SETTINGS ...).

2. Older-ClickHouse compatibility (uncovered while backporting): on CH 23.x/24.x the same CREATE TABLE ... AS source ENGINE = ... form does not implicitly inherit PARTITION BY / ORDER BY from the source. Fixed in master by !240627 (merged) (specify them explicitly). 19.0 only runs CH 25/26 so this wasn't strictly necessary here, but kept identical to master to maintain the strict-subset invariant for future cherry-picks.

This branch contains three cherry-picks from master:

commit	from MR	purpose
ca503ffb	!240219 (merged)	Initial fix: clone source structure via CREATE TABLE … AS source
d811b49f	!240627 (merged)	Explicit PARTITION BY / ORDER BY for CH 23.x/24.x compatibility
b4f205d5	!240798 (merged)	drop_tmp_table DETACH+DROP path on CH 23.x, version-agnostic spec, dedup-gate threshold fix (CH 24.8+), self-healing create_tmp_table on retry, and a .pluck('name') rubocop fix in the spec for rubocop-rails 2.26+.

No conflicts. All three apply cleanly.

References

• Issue: #593129 (closed) (severity::2, priority::2, Customer Interest, GitLab Dedicated)
• Master MRs: !240219 (merged), !240627 (merged), !240798 (merged)
• Customer ticket: ZD#700951

MR acceptance checklist

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• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
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• 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.

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