fix: Return NULL for empty BIT_XOR sliding windows

[geoffreyclaude]

Which issue does this PR close?

No existing issue. The SQL regression below reproduces the bug on current main.

Rationale for this change

BIT_XOR returns 0 instead of NULL after the last non-null value leaves a sliding window. For values 7, NULL, 9, a window containing only the preceding row produces NULL, 7, 0; the correct results are NULL, 7, NULL.

SELECT id,
       bit_xor(v) OVER (
           ORDER BY id ROWS BETWEEN 1 PRECEDING AND 1 PRECEDING
       ) AS x
FROM (VALUES (1, CAST(7 AS BIGINT)),
             (2, CAST(NULL AS BIGINT)),
             (3, CAST(9 AS BIGINT))) AS t(id, v)
ORDER BY id;

What changes are included in this PR?

Add a sliding BIT_XOR accumulator that tracks the remaining non-null count alongside the XOR value. This distinguishes an empty aggregate from a nonempty aggregate whose values cancel to zero.

The sliding factory supplies this accumulator. The ordinary accumulator no longer advertises retraction support. Ordinary scalar and grouped aggregation retain their existing one-field state and groups implementation.

What is the testing strategy for this PR?

Rust tests cover the public sliding factory's zero/NULL transitions, merging partial states with correct input counts, and the ordinary/DISTINCT factory contracts.

bit_xor_sliding_window.slt exercises all eight integer types across batch boundaries, including empty and all-null frames, actual zero, and cancelling values.

Are there any user-facing changes?

Sliding BIT_XOR windows return NULL after all non-null inputs leave. Callers that need retraction should request the sliding accumulator; the ordinary factory reports that it does not support retraction.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 72.51908% with 36 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.72%. Comparing base (db83fcc) to head (436baf5).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
...tafusion/functions-aggregate/src/bit_and_or_xor.rs	72.51%	6 Missing and 30 partials ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             main   #26095     +/-   ##
=========================================
  Coverage   82.72%   82.72%             
=========================================
  Files        1147     1147             
  Lines      448179   449287   +1108     
  Branches   448179   449287   +1108     
=========================================
+ Hits       370754   371674    +920     
- Misses      54895    54935     +40     
- Partials    22530    22678    +148     

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