fix(optimizer): check IN casts and avoid nullability warning probes

[willlling]

Summary

Fix two optimizer checks that evaluate the wrong thing:

• IN predicate deduction checked a casted scalar against the IN-list container's null type and rejected valid queries with error 5083. Check each list element instead.
• Nullability inference evaluated warning/error-capable constants while probing their properties. Keep nullability unknown for these expressions and preserve their execution-time evaluation.

A minimal IN trigger is:

CREATE TABLE t1(c DECIMAL(10));
CREATE TABLE t2(c DECIMAL(20));
SELECT t1.c, t2.c FROM t1, t2
WHERE t1.c = t2.c AND t2.c IN (10, 20);

CI follow-up and dependency

The completed CI run 34937566348 reached mysqltest after its earlier checkout failure. Its only failing case was sqlancer_optimizer_regressions: count 9 plus four X3 decimal-conversion warnings. Both the fixed baseline dc88204e5 and this PR's initial head reproduced that result with identical full rejects.

This branch now merges the pinned dependency #1392 at d80905ab86fc532ac60b15b4761c9adb4ed81931. That dependency is not yet merged into the base branch; its shared implementation is retained through the merge, not reimplemented here.

The dependency removes two speculative warnings. This PR's is_const_expr_not_null guard removes another speculative warning by keeping nullability unknown for warning/error-capable expressions. The remaining warning comes from actual filter execution (ObOperator::filter_batch_rows); the no-rewrite control also reports it. The resolved result-file conflict therefore retains exactly one X3 warning. Extra or missing diagnostics still fail.

Current combined commit: 55a69c3dcc1676b88fbbe2aae3c589417f5796f4. A fresh release configuration regenerated the embedded revision for that exact commit. The running process executable SHA256 matched the built binary. Local validation used the CI init.sql/init_user.sql and admin/test context, with an isolated instance.

Validation

• Current combined release build: PASS.
• Both original complete 5083 regression cases: PASS with unchanged expected output.
• predicate_deduce_in_cast: PASS (decimal precision, binary/text, derived-table collation; EXPLAIN and SELECT).
• sqlancer_optimizer_regressions: PASS, count 9 and exactly one required runtime warning.
• git diff --check: PASS.
• Current commit CI 35050028619: SUCCESS. Setup, all four mysqltest slices, collect, and all other PR checks passed.

Runtime failure stacks and a correction confined to the IN helper attribute the 5083 regression to #1379 (402f0e25b95006747f31477a78907e0a66147b83); no historical parent-binary bisect was performed. The additional CI warning failure predates this PR; its introducing commit has not been established.

Links

• SEEK-593
• SEEK-586

Fixed 2026-09-17 baseline verification

SEEK-609 verifies the net fixes from pinned #1392 d80905ab86fc532ac60b15b4761c9adb4ed81931 and this combined head 55a69c3dcc1676b88fbbe2aae3c589417f5796f4 on baseline 8a8d6f5a8dd48cdbf859305b3f7eb050149a5bd1. The isolated verification commit is c98a258c61c45934c932349784290bd2edda7862 (full revision and binary hash in the SEEK-609 evidence attachment). It excludes unrelated PR1394/bigtrans changes.

The internal test MR64 must require exactly one runtime X3 warning. Its old four-warning expectation matches the unfixed product (EXPLAIN 3, SELECT 4) and is not the corrected semantic oracle. Original SQL and warning checks remain unchanged.

Related: SEEK-604, SEEK-609.

Local validation on that fixed-baseline combination: 18/18 complete cases PASS, including all 12 original warning cases, both original IN/deduction cases, SQLancer (counts 9/5/5/1, exactly one X3 warning), predicate_deduce_in_cast, uuid_having_warning, and window_function.sqlancer_const_prop_cast. Original query and all-block NO_REWRITE phase probes both produced EXPLAIN 0 / SELECT 1 warning, count 9. Binary and running executable SHA256: d6144cd64fbadabf56feed14c8ab3c382a2fe5d1240b633ee0df38b3f993ab79. Initial -j80 build lost one geo compiler process (Killed); incremental -j8 build succeeded. No additional product fix was needed; this verification does not establish historical warning first-bad. MR64 is updated on its original branch to 74261325ea064c75597f8272c66cbba42800ac9a; no PR/MR was merged.