release-26.2: sql: fix partial index data loss / phantom rows during update

[fqazi]

Backport 1/1 commits from #166123 on behalf of @fqazi.

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sql: fix partial index data loss / phantom rows during update

This commit fixes a bug on tables with multiple column families where a
concurrent update that does not overlap with a partial index's column
family could cause the partial index to write a NULL instead of the
actual data, or incorrectly add phantom rows to a temporary index
during a schema change backfill.

This bug was previously masked on default (single-column-family) tables
because an update to any column causes the optimizer to conservatively
fetch all columns in that family. However, with multiple column families,
two normalization rules in the optimizer caused issues:

1. PruneMutationFetchCols: If an update does not change any column
   associated with an index, the optimizer avoids fetching those
   columns. This causes the execution layer to see NULLs for the
   unfetched columns.
2. SimplifyPartialIndexProjections: If an update does not change
   any column associated with a partial index, the optimizer
   simplifies the partial index predicate evaluation to FALSE.
   This causes the execution layer to skip writes to the index.

During a schema change backfill, the execution layer's updater must
unconditionally write complete index entries to temporary (mutating)
indexes for any concurrent update, even if the index's columns are
unchanged. This ensures the backfill merger has a complete snapshot
to correctly reconcile the final index. If columns are pruned (Rule 1)
or writes are simplified away (Rule 2), the temporary index receives
incomplete entries (NULLs) or misses the update entirely.

Furthermore, missing columns can lead to phantom rows if the partial
index predicate evaluates to TRUE when given a NULL value (e.g.,
WHERE val IS NULL).

This change ensures the optimizer always fetches the required
columns and avoids simplifying predicate evaluation if the index
is a mutation index, correctly propagating the full row state to the
execution layer.

Fixes: #166122

Release note (bug fix): Fixed a bug where concurrent updates to a table
using multiple column families during a partial index creation could
result in data loss, incorrect NULL values, or validation failures in
the resulting index.

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Release justification: addresses a bug that can lead to partial indexes that are corrupt or fail to construct in the face of concurrent updates.

[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.

[cockroach-teamcity]

This change is 

[michae2]

@michae2 reviewed 4 files and all commit messages, and made 1 comment.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on mw5h and yuzefovich).