fix: preserve NULL semantics in bitwise xor simplification

[lyne7-sc]

Which issue does this PR close?

• Part of Expression simplifications may produce incorrect results with NULLs #24246 .

Rationale for this change

Bitwise XOR simplifications cancelled repeated nullable operands, which could incorrectly produce a non-NULL result when the operand was NULL.

For example:

SELECT
  i XOR i,
  (i XOR 7) XOR i,
  i XOR (7 XOR i)
FROM (VALUES (NULL::INT)) AS t(i);

These expressions should all return NULL, but simplification could replace them with 0, 7, and 7.

What changes are included in this PR?

Only cancel repeated XOR operands when the removed operand is non-nullable.

Are these changes tested?

Yes. Added sqllogictests covering results.

Are there any user-facing changes?

Yes. Bitwise XOR expressions with repeated nullable operands now correctly preserve NULLs.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.01%. Comparing base (918013e) to head (d1a84e7).
⚠️ Report is 22 commits behind head on main.

Files with missing lines	Patch %	Lines
...imizer/src/simplify_expressions/expr_simplifier.rs	96.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24248      +/-   ##
==========================================
- Coverage   81.06%   81.01%   -0.06%     
==========================================
  Files        1106     1106              
  Lines      381891   384036    +2145     
  Branches   381891   384036    +2145     
==========================================
+ Hits       309578   311121    +1543     
- Misses      54034    54576     +542     
- Partials    18279    18339      +60     

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