release-25.2: sql: do not rewrite UDF body statement slice while assigning placeholders

[blathers-crl]

Backport 1/1 commits from #147187 on behalf of @DrewKimball.

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Previously, we accidentally modified the original slice that contains the body statements of a UDF while copying it during the placeholder assignment step. As a result, constant folding that occurred in one session could become visible in the query plan cache, causing incorrect results. This commit fixes the bug by copying the slice as well as the body statements.

This bug only applied to prepared statements, since we don't add plans with stable expressions to the plan cache outside of the prepare path.

Fixes #147186

Release note (bug fix): Fixed a bug that could cause stable expressions to be folded in cached query plans. The bug could cause stable expressions like current_setting to return the wrong result if used in a prepared statement. The bug was introduced in point releases v23.2.22, v24.1.14, v24.3.9, and v25.1.2, and the v25.2 alpha.

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Release justification: critical fix for bug introduced in earlier backport.

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

[cockroach-teamcity]

This change is 

[michae2]

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