release-25.1: sql: improve observability of index merge timestamp

[spilchen]

Backport 1/1 commits from #142917.

/cc @cockroachdb/release

---

When building an index, the merge phase copies any new rows added since the index build started. A timestamp is used to set an upper bound for this scan. We suspect that the chosen timestamp may be omitting some rows.

Our theory is that this happens if new rows were inserted before the merge phase but were committed with a timestamp after the merge. This discrepancy could be due to clock skew between nodes. However, attempts to reproduce this issue via unit tests were unsuccessful, so this remains a hypothesis.

To address this, this change includes:

• An adjustment to the merge timestamp to account for potential clock skew.
• Additional logging of the merge timestamp chosen and the timestamps observed on each node when draining leased descriptors. These logs will help track the timestamp across nodes during the merge phase.
• Treat index validation errors for non-unique indexes as assertion failures. We previously treated as a duplicate key error, which was very confusing because duplicates are allowed for non-unique indexes.

Epic: none
Release note: none
Closes: #142861
Closes #143050

Release justification: Bug fix for customer problem.

[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.
• Backports should not change on-disk formats or node communication protocols.
• Backports should not add new functionality (except as defined
  here).
• Backports must not add, edit, or otherwise modify cluster versions; or add version gates.
• All backports must be reviewed by the owning areas TL. For more information as to how that review should be conducted, please consult the backport
  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).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters. State changes must be further protected such that nodes running old binaries will not be negatively impacted by the new state (with a mixed version test added).
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.
• Your backport must be accompanied by a post to the appropriate Slack
  channel (#db-backports-point-releases or #db-backports-XX-X-release) for awareness and discussion.

Also, please add a brief release justification to the body of your PR to justify this
backport.

[cockroach-teamcity]

This change is