release-25.4: backfill: add a separate control for number of vectors merged per batch

[blathers-crl]

Backport 1/1 commits from #155284 on behalf of @mw5h.

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Add the 'bulkio.index_backfill.vector_merge_batch_size', which controls the number of rows to merge into vector indexes per transaction while the index is being created. This is analogous to the 'bulkio.index_backfill.merge_batch_size' setting, but it only applies when the target index is a vector index. By default, 3 vectors will be merged per transaction to reduce contention with fixup tasks.

Fixes: #155283
Release note (sql change): Added the
bulkio.index_backfill.vector_merge_batch_size cluster setting to control how many vectors to merge into a vector index per transaction during create operations. By default, this defaults to 3.

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Release justification: This is needed for acceptable performance of a feature that is slated for 25.4 GA.

[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 

[michae2]

@michae2 reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @DrewKimball, @fqazi, and @rafiss)