[4.1-dev] Fix nullable IN-list runtime semantics

[LeftHandCold]

What type of PR is this?

• API-change
• BUG
• Improvement
• Documentation
• Feature
• Test and CI
• Code Refactoring

Which issue(s) this PR fixes:

issue #25346

Follow-up to #25426.

What this PR does / why we need it:

The previous zonemap fix made block pruning conservative for nullable IN lists, but the SQL path can still fail before row-level filtering. Planner constant folding converts IN list literals into a folded vector. Vector.InplaceSortAndCompact() sorts the vector payload without moving the null bitmap, so ('keep', NULL) can become a vector where the null bitmap no longer points at the same value. The runtime in operator then skips the wrong RHS slot while building its hash map, and WHERE k IN ('keep', NULL) can filter out matching keep rows.

This PR fixes the full path for 4.1-dev:

• keeps folded nullable IN-list vectors unsorted/uncompacted so values and null bitmap stay aligned;
• makes vectorized IN / NOT IN return SQL UNKNOWN for no-match rows when the RHS contains NULL;
• keeps nullable folded vectors conservative in zonemap/readutil pruning instead of relying on sorted first/last entries;
• adds tests for operator three-valued logic, folded predicate execution, nullable folded vectors in zonemap pruning, and varchar/fixed-type nullable zonemap AnyIn.

Validated with the issue-shaped SQL on a dev node:

SELECT COUNT(*) FROM t WHERE k IN ('keep', NULL);
-- 2

SELECT COUNT(*) FROM t WHERE k NOT IN ('zzz', NULL);
-- 0

Local checks:

go test ./pkg/sql/plan/function -count=1
go test ./pkg/sql/colexec -count=1
go test ./pkg/vm/engine/readutil -count=1
go test ./pkg/vm/engine/tae/index -count=1
go test ./pkg/sql/plan/rule -count=1
go test ./pkg/sql/plan -run 'Test.*Fold|Test.*In|Test.*Filter' -count=1

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