release-25.1: workload/schemachange: improve diagnostics for unique constraint failures

[blathers-crl]

Backport 1/1 commits from #142983 on behalf of @spilchen.

/cc @cockroachdb/release

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Previously, when adding a unique constraint to a table, failures due to duplicate key checks were diagnosed by comparing table row counts against a DISTINCT check. However, when this failed, it provided little insight into the cause.

This change improves diagnostics by modifying the query to explicitly extract the components of the check, ensuring they are included in test logs. This additional context will make debugging easier if the issue recurs.

Closes #142458

Epic: none
Release note: none

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Release justification: test only fix

[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.
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• Backports should not add new functionality (except as defined
  here).
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  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).
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• 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