fix(optimizer): don't expand aggregate aliases into GROUP BY

[fivetran-kwoodbeck]

qualify_columns can replace a GROUP BY reference with an aggregate alias's definition, nesting an aggregate inside GROUP BY which gives invalid SQL and is also non idempotent. Fixed by ensuring aggregate alias is never expanded when is_group_by.

Sample input:

SELECT MAX(x) AS x FROM a AS a JOIN b AS b ON a.id = b.id GROUP BY UPPER(x)

Previous output:

SELECT MAX(x) AS x FROM a AS a JOIN b AS b ON a.id = b.id GROUP BY UPPER(MAX(x))

Note: GROUP BY UPPER(x) became GROUP BY UPPER(MAX(x)) which is wrong.

Output after Patch:

SELECT MAX(x) AS x FROM a AS a JOIN b AS b ON a.id = b.id GROUP BY UPPER(x)

[github-actions]

SQLGlot Integration Test Results

✅ All tests passed

Comparing:

• this branch (sqlglot:optimizer/fix-qualify-group-by-alias-collision @ sqlglot 597f605)
• baseline (main @ sqlglot feec46e)

Overall

main: 192414 total, 153558 passed (pass rate: 79.8%)

sqlglot:optimizer/fix-qualify-group-by-alias-collision: 180220 total, 142406 passed (pass rate: 79.0%)

Transitions:
No change

Dialect pair changes: 0 previous results not found, 3 current results not found

✅ All tests passed