release-25.2: opt: fix trigger+computed column internal error

[blathers-crl]

Backport 1/1 commits from #154789 on behalf of @mgartner.

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This commit fixes a bug that was caused by incorrectly modifying the set
and list of target columns when re-projecting computed columns after
building a BEFORE trigger.

Fixes #154672

Release note (bug fix): A bug has been fixed that caused internal errors
for INSERT .. ON CONFLICT .. DO UPDATE statements when the target
table had both a computed column and a BEFORE trigger. This bug has
been present since triggers were introduced in v24.3.0.

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Release justification: Low-risk bug fix.

[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.

[blathers-crl]

❌ PR #155075 does not comply with backport policy

Confidence: high
Explanation: The PR details indicate that this change is a bug fix addressing an issue with internal errors when certain conditions involving triggers and computed columns are met in INSERT statements. Specifically, these errors occur with the combination of computed columns and BEFORE triggers. The mention of fixing an error that previously allowed incorrect execution paths suggests the bug might be critical. However, there is no explicit statement affirming it as a critical bug under the specified criteria of stability, security, data corruption/loss, significant performance regressions, incorrect results/suboptimal performance, or debugging capabilities. Without a complete Release justification provided in the PR body or a critical bug categorization under the listed policy criteria, the backport does not comply. Also, there is no indication of feature flag implementation for this change.
Recommendation: Add a justification as to why this is a critical bug fix that can be safely backported without a feature flag or consider gating the change behind a feature flag.

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[cockroach-teamcity]

This change is