opt: cast inlined constant to the variable's type in InlineConstVar

[shafi-VM]

InlineConstVar replaces a variable that a filter restricts to a constant (e.g. x in x = 4 AND x IN (1, 2, 3, 4)) with that constant elsewhere in the filters. The constant is matched against the variable using Equivalent rather than Identical, so a constant whose type is equivalent but not identical to the variable's type could be substituted directly—for example, a STRING constant for a NAME column. This changes the result of type-sensitive expressions: pg_typeof(n) became pg_typeof('hello'), returning text instead of name, so a filter like pg_typeof(n)::TEXT = 'name' folded to false and the row was incorrectly dropped.

Cast the inlined constant to the variable's type when the two types are not identical, so type-sensitive expressions continue to observe the original type. The common case, where the types are identical, is unaffected—no cast is added.

Fixes #170544

Epic: none

Release note (bug fix): Fixed a bug that could cause a query to return incorrect results when a column was compared to a constant of an equivalent but different type (for example, a NAME column compared to a string literal) and a type-sensitive expression such as pg_typeof was applied to the same column. The optimizer could substitute the constant using the wrong type, changing the result of the type-sensitive expression

[blathers-crl]

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[mw5h]

This is looking really good. I have a couple of requested changes around testing, but this is a nice clean fix.

[blathers-crl]

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