fix: prevent panic and incorrect results for COUNT with ORDER BY

[geoffreyclaude]

Which issue does this PR close?

None; this is a minor bug fix.

Rationale for this change

COUNT(... ORDER BY ...) can panic when used with GROUP BY, or return a count that is too small when an ordering column contains nulls.

SELECT COUNT(a + 0 ORDER BY b)
FROM (VALUES (1, NULL), (2, 20)) AS t(a, b);

This query should return 2, because both values being counted are non-null. Previously, it returned 1: the null in ordering column b incorrectly excluded a row from the count. Grouping a single-argument count with ORDER BY could instead trigger an assertion failure.

COUNT does not depend on input order. However, it inherited the default order-sensitive behavior, which passed ordering keys to its accumulators as additional arguments and could introduce unnecessary sorting.

What changes are included in this PR?

Mark Count as order-insensitive so that ordering keys are excluded from accumulator inputs and no sorting is required solely for the aggregate's ORDER BY.

What is the testing strategy for this PR?

Regression tests in aggregate.slt cover grouped, multi-argument, and non-grouped counts. The data includes both a null ordering key, which must not affect the count, and a null counted argument, which must still be excluded. The non-grouped test uses a + 0 to prevent the count from being answered from column statistics.

An EXPLAIN assertion verifies that the count does not require a sort.

Are there any user-facing changes?

COUNT(... ORDER BY ...) returns the correct count without panicking. Nulls in counted arguments retain their usual behavior; nulls in ordering keys no longer incorrectly exclude rows.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.67%. Comparing base (262936e) to head (0c5548e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24997      +/-   ##
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- Coverage   81.67%   81.67%   -0.01%     
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  Files        1126     1126              
  Lines      414842   414845       +3     
  Branches   414842   414845       +3     
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- Hits       338841   338834       -7     
- Misses      56070    56076       +6     
- Partials    19931    19935       +4     

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