fix: preserve NULL semantics when simplifying col ~ '.*'

[viirya]

Which issue does this PR close?

• Closes Expression simplification of col ~ '.*' to col IS NOT NULL drops NULL semantics #24379.

Rationale for this change

simplify_regex_expr rewrites col ~ '.*' to col IS NOT NULL. For a NULL input that returns false, but NULL ~ '.*' is NULL under three-valued logic — so the rewrite produces wrong results in a projection context:

SELECT s, s ~ '.*' FROM (VALUES (CAST(NULL AS VARCHAR)), ('x')) t(s);
-- NULL row currently returns `false`; it should be NULL

The !~ (RegexNotMatch) branch of the same rule is already NULL-aware (col IS NULL AND NULL); only the ~ branch dropped the NULL.

What changes are included in this PR?

• Rewrite col ~ '.*' to col IS NOT NULL OR NULL — true for a non-NULL string, NULL for a NULL input.
• In a WHERE filter both FALSE and NULL reject the row, so filter results are unchanged; only the plan text and projection-context values differ. Existing filter-plan expectations in simplify_expr.slt and the test_simplify_regex_special_cases unit test are updated accordingly, and a projection regression test is added.

Are these changes tested?

Yes.

• New projection regression test in simplify_expr.slt asserting col ~ '.*' returns true/true/NULL for 'foo'/''/NULL.
• Updated the two filter-context plan expectations (logical + physical) that previously encoded the IS NOT NULL rewrite.
• simplify_expr.slt, the regexp/* SLTs, and the optimizer simplify unit tests all pass.

Are there any user-facing changes?

col ~ '.*' in a projection now returns NULL for a NULL input instead of false, matching SQL semantics. No API changes.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.18%. Comparing base (1b67f2e) to head (c4ecaef).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24380      +/-   ##
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- Coverage   81.19%   81.18%   -0.01%     
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  Files        1110     1110              
  Lines      388616   388740     +124     
  Branches   388616   388740     +124     
==========================================
+ Hits       315529   315617      +88     
- Misses      54506    54535      +29     
- Partials    18581    18588       +7     

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[alamb]

Thank you @viirya

[viirya]

Thank you @alamb for review!