release-25.3: sql/opt_catalog: Use index offset as tombstone index ordinal

[blathers-crl]

Backport 1/1 commits from #156105 on behalf of @mw5h.

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Previously, when the optimizer stored the ordinal of an index to use for writing tombstones in non-SERIALIZABLE transactions, it would call the Ordinal() method, which returns the value from the table's descriptor. We would later use this value as an input to the Index() method to retrieve the index in question. However, Ordinal() can differ from the catalog's view of the schema during schema change, resulting in sometimes getting the wrong index ordinal from the optimizer's point of view, sometimes resulting in an out of bounds error.

This patch switches the ordinal we save to be the actual index used when creating the optTable, which feels janky, but is at least correct.

Fixes: #151477
Release note (bug fix): A bug which could occasionally cause DML on regional by row tables with unique indexes that don't reference the region to fail under READ-COMMITTED isolation has been corrected.

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Release justification: Simple fix for problem found in testing.

[blathers-crl]

Thanks for opening a backport.

Before merging, please confirm that the change does not break backwards compatibility and otherwise complies with the backport policy. Include a brief release justification in the PR description explaining why the backport is appropriate. All backports must be reviewed by the TL for the owning area. While the stricter LTS policy does not yet apply, please exercise judgment and consider gating non-critical changes behind a disabled-by-default feature flag when appropriate.

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

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[cockroach-teamcity]

This change is