Fix COUNT(DISTINCT ...) collision for multi-column comma values

[chiliec]

What

Fixes a COUNT(DISTINCT a, b) correctness bug where distinct multi-column tuples are counted as one. Reported downstream at dolthub/dolt#11562.

CREATE TABLE t(id INT PRIMARY KEY, a VARCHAR(16), b VARCHAR(16));
INSERT INTO t VALUES (1, 'a,', 'b'), (2, 'a', ',b');
SELECT COUNT(DISTINCT a, b) FROM t;   -- MySQL: 2, was returning 1

Fix

The multi-column path in countDistinctBuffer.Update built its hash key by joining each converted value with a , separator (str += vv + ","). That encoding is not injective: ('a,','b') and ('a',',b') both produce "a,,b,", so distinct tuples collide into one hash bucket. Length-prefixing each value ("%d:%s") makes the encoding self-delimiting, so different tuples always produce different keys.

Tests

Added TestCountDistinctEvalMultiColumn in count_test.go using the reporter's exact tuples.

Validation

go test -tags gms_pure_go ./sql/expression/function/aggregation/

• New test: RED on main (returns 1, expected 2), GREEN with the fix.
• Full aggregation package suite passes.
• gofmt/goimports -local github.com/dolthub/go-mysql-server clean.

Happy to adjust.