release-25.2: execbuilder: fix a silly bug around finding the most recent full stat

[yuzefovich]

Backport 1/1 commits from #148329 on behalf of @yuzefovich.

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This commit fixes a bug that was added in 3bc0992. In short, in a complicated logical expression we had v1 && v2 || v3 when we wanted v1 && (v2 || v3). This allowed us to hit an index of bounds when evaluating v3, which in this concrete case could mean that we iterated over all stats and didn't find any full ones.

There are two other places with similar conditionals, and they have the correct usage of parenthesis, so I didn't change the structure overall. I did find that we want to also skip the merged stat, depending on the session variable.

Also the bug seems quite difficult to hit, so I omitted the release note.

Fixes: #148316.

Release note: None

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Release justification: critical bug fix: we've seen this bug result in a failure to take a backup (#148316) since it can error out an internal query used by the backup (another example is #148414 with read-role-with-memberships internal query). In the extreme, failure to backup data can result in a data loss.

[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.
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If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

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

[cockroach-teamcity]

This change is 

[mgartner]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @michae2)

[michae2]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! 2 of 0 LGTMs obtained (waiting on @yuzefovich)

[yuzefovich]

TFTRs!