fix: Decide NOT IN null-aware joins from expression nullability, not just column nullability

[namanjain24-sudo]

Which issue does this PR close?

• Closes Wrong results: nullable constant NOT IN (subquery) returns every row when the subquery column is NOT NULL #25473.
• Closes Wrong results: NOT IN (subquery) ignores NULLs from a nullable outer expression over NOT NULL columns #25474.

Rationale for this change

NOT IN (subquery) decides whether it needs null-aware join semantics by
checking if any column in the join filter is nullable. That misses a NULL
constant (NULL NOT IN (...)) or an expression that is nullable on its own
(CASE without ELSE, NULLIF) even when every column it reads is NOT NULL. In both cases a row that should be UNKNOWN (and dropped from the
result) is silently kept.

What changes are included in this PR?

join_keys_may_be_null in decorrelate_predicate_subquery.rs now checks the
nullability of each equality operand as an expression (via ExprSchemable),
not just the columns it references, falling back to "may be null" when that
can't be resolved to one side.

What is the testing strategy for this PR?

• Two optimizer unit tests: a NULL constant against a NOT NULL subquery
  column, and a CASE expression over NOT NULL columns, both asserting the
  resulting join is null-aware.
• New cases in null_aware_anti_join.slt covering both issues' tables,
  including the OR mark-join form and EXPLAIN assertions. Confirmed each
  new query fails on main and passes with this change.
• cargo test -p datafusion-optimizer and the full default sqllogictest
  suite (521 files) pass. Checked the TPC-H Q16 plan (the only TPC-H query
  using this path) is unchanged.

Are there any user-facing changes?

NOT IN subqueries now return correct (fewer) rows for these two cases.
No public API changes.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 71.66667% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.44%. Comparing base (ebdb657) to head (0fa605d).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...on/optimizer/src/decorrelate_predicate_subquery.rs	71.66%	4 Missing and 13 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25641      +/-   ##
==========================================
- Coverage   82.45%   82.44%   -0.01%     
==========================================
  Files        1140     1140              
  Lines      436434   436486      +52     
  Branches   436434   436486      +52     
==========================================
+ Hits       359840   359859      +19     
- Misses      54840    54855      +15     
- Partials    21754    21772      +18     

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