release-24.2: sql: Check for concurrent DSC job in legacy schema changer

[blathers-crl]

Backport 1/1 commits from #137868 on behalf of @spilchen.

/cc @cockroachdb/release

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Running the legacy schema changer and the declarative schema changer concurrently can cause issues due to their different approaches to updating descriptors. Normally we have checks to prevent the legacy schema changer from running in such scenarios, timing issues persisted—particularly between ALTER VIEW .. RENAME (legacy) and DROP VIEW (DSC). In these cases, the view rename could delete the descriptor being processed by the drop view operation.

This change addresses the timing issue by adding a check for an active DSC job at the start of the legacy schema changer job. With this fix, the issue could no longer be reproduced, whereas it was consistently reproducible before.

Epic: none
Closes: #137487
Closes: #137828
Release note (bug fix): Fixed a timing issue between ALTER VIEW .. RENAME and DROP VIEW that caused repeated failures in the DROP VIEW job.

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Release justification:

[blathers-crl]

Thanks for opening a backport.

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[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]

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