release-25.3: sql: fix identity computed columns with projection with generic plans

[ZhouXing19]

Backport 1/1 commit from #162036

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Previously, when a computed column was defined as a identical to another column (e.g., vcol AS (col)), and a generic plan was used with a lookup join on an index containing that column (vcol), the query would fail with "cannot map variable %d to an indexed var".

This occurred because the Factory.Replace function doesn't invoke the replacement callback for leaf nodes like VariableExpr, as it only traverses children. Thus, with the computed column expression a simple VariableExpr, table column references weren't getting replaced with their corresponding placeholder columns. As a result, when constructing the projection, we went through the MergeProjectWithValues rule incorrectly.

This fix adds special handling in the constraint builder to directly apply the replacement function when the computed column expression is a simple VariableExpr, ensuring table columns are properly replaced with their corresponding placeholder columns.

Fixes #161978

Release note (bug fix): Fixed an error that occurred when using generic plan that generates a lookup join on indexes containing identity computed columns.

Release justification: Resolve a correctness issue where queries using identity computed columns with generic plans would fail with an error, preventing customers from executing valid queries.

[blathers-crl]

Thanks for opening a backport.

Before merging, please confirm that the change does not break backwards compatibility and otherwise complies with the backport policy. Include a brief release justification in the PR description explaining why the backport is appropriate. All backports must be reviewed by the TL for the owning area. While the stricter LTS policy does not yet apply, please exercise judgment and consider gating non-critical changes behind a disabled-by-default feature flag when appropriate.

[cockroach-teamcity]

This change is