release-24.3.1-rc: optbuilder: always select tombstone index using the index ordinal

[blathers-crl]

Backport 1/1 commits from #137361 on behalf of @mw5h.

/cc @cockroachdb/release

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Previously, the index to which tombstones would be written to enforce uniqueness was chosen using the ordinal of uniqueness constraint, which happens to be the same as the index ordinal if no non-unique indexes precede the unique indexes on a relation. However, if a non-unique index precedes a unique index, we will erroneously apply the uniqueness check to that index. In most cases, this will cause queries to fail on an assertion within the row helper code.

However, a mixture of serializable and non-serializable mutations and some sequences of index additions and non-serializable mutations can cause uniqueness to be violated. This impacts all regional by row tables with uniqueness constraints that do not include the region which are mutated under non-serializable isolations.

This patch plumbs through the index ordinal so that the proper index will be checked for uniqueness in all cases.

Fixes: #137341
Release note (bug fix): Regional by row tables with uniqueness constraints where the region is not part of those uniqueness constraints and which also contain non-unique indices will now have that uniqueness properly enforced when modified at READ-COMMITTED isolation. This bug was introduced in
release 24.3.0.

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Release justification: Fix for potential data corruption bug.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
• Backports should not break backwards-compatibility.
• Backports should change as little code as possible.
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• Backports should not add new functionality (except as defined
  here).
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  policy.

If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
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Also, please add a brief release justification to the body of your PR to justify this
backport.

[cockroach-teamcity]

This change is 

[michae2]

Nice!

Reviewed 5 of 5 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @mgartner, @mw5h, and @rytaft)

[mgartner]

Don't forget to add a release justification to the PR description.

Reviewed 5 of 5 files at r1, all commit messages.
Reviewable status: complete! 2 of 0 LGTMs obtained (waiting on @mw5h and @rytaft)