release-25.2: sql: block ALTER TABLE LOCALITY to RBR with sql_safe_updates

[rafiss]

Backport 1/1 commits from #151423.

/cc @cockroachdb/release

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A bug was recently discovered that causes UPDATE and DELETE operations to fail on a table while it is being converted to REGIONAL BY ROW.

To protect against this, this patch blocks the ALTER TABLE LOCALiTY statement if the sql_safe_updates flag is on. The error hint provides a workaround to safely convert the locality.

fixes #150945
Release note (sql change): When sql_safe_updates is enabled, the ALTER TABLE LOCALITY statement will now be blocked when trying to convert an existing table to REGIONAL BY ROW, unless a region column has already been added to the table. This protects against undesired behavior that caused UPDATE or DELETE queries to fail against the table while the locality change is in progress.

Release justification: Add a safety check for an operation that can cause DML to stop working, with instructions on how to proceed safely.

[blathers-crl]

Thanks for opening a backport.

Before merging, please confirm that it falls into one of the following categories (select one):

• Non-production code changes. Includes test-only changes, build system changes, etc.
• Fixes for serious issues. Defined in the policy as correctness, stability, or security issues, data corruption/loss, significant performance regressions, breaking working and widely used functionality, or an inability to detect and debug production issues.
• Other approved changes. These changes must be gated behind a disabled-by-default feature flag unless there is a strong justification not to.

Add a brief release justification to the PR description explaining your selection.

Also, confirm that the change does not break backward compatibility and complies with all aspects of the backport policy.

All backports must be reviewed by the TL and EM for the owning area.

[cockroach-teamcity]

This change is 

[blathers-crl]

✅ PR #152600 is compliant with backport policy

Confidence: high
Critical bug criteria met: [Stability or security issues Bugs that can cause the DB to return incorrect results or result in suboptimal performance]
Feature flag detected: Yes
Backward compatible: true
Explanation: The pull request is compliant with the backport policy for a number of reasons. Firstly, the PR includes a 'Release justification' which effectively states that the change adds a safety check to prevent DML failures during REGIONAL BY ROW conversions when sql_safe_updates is enabled, thereby addressing a significant bug that could affect database stability and correctness. This aligns with the critical bug criteria, specifically those related to stability and potentially incorrect results. Secondly, the PR is backward compatible as it doesn't remove any version gates or existing functionality, instead it adds conditions to manage specific alterations more safely. The actual change in 'pkg/sql/alter_table_locality.go' directly implements this safety feature effectively gating the operation behind existing session settings, thus satisfying feature flag requirements inherently. Finally, all changes including those in test files are appropriate, targeting correct functionality coverage and ensuring the feature's impact is well-tested without affecting non-test environments.

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