release-25.2: changefeedccl: fix unwatched column families memory monitoring bug

[andyyang890]

Backport 1/1 commits from #154802.

/cc @cockroachdb/release

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This patch fixes a bug where the memory monitor wouldn't reclaim the
memory allocated to events corresponding to unwatched column families
for a changefeed that targets only a subset of a table's families.

Fixes #154776

Release note (bug fix): A bug where a changefeed targeting only a subset
of a table's column families could become stuck has been fixed.

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Release justification: fix for high-severity bug

[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 #155026 is compliant with backport policy

Confidence: high
Critical bug criteria met: [Bugs that can cause the DB to return incorrect results or result in suboptimal performance]
Backward compatible: true
Explanation: The PR backports a bug fix geared towards resolving an issue where a changefeed targeting only a subset of a table's column families could become stuck. The purpose of this fix is clearly to address a significant performance regression and prevent potential data issues related to unprocessed or stuck changefeeds. This complies with the critical bug criteria specified in the backport policy for 'Bugs that can cause the DB to return incorrect results or result in suboptimal performance.'

Additionally, the PR includes a 'Release justification: fix for high-severity bug' in its body, satisfying the release justification clause that exempts it from the standard backport policy requirements regarding feature flags. The files changed do not include non-production files and do not involve backward compatibility issues or removal of version gates. The evidence provided in the PR description and the specific changes justify the backport based on the current policy.

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