A fixed GROUP BY column is incorrectly used to decide when groups are complete #25018

Describe the bug

When a WHERE condition limits a GROUP BY column to one value, we have the pieces to know that the column is fixed. However, aggs currently treats that column as if a change in its value could show that earlier groups are complete.

Since the column is fixed because of the filter, it cannot provide such a boundary.

eg:

SELECT key, market, COUNT(*)
FROM t
WHERE market = 'US'
GROUP BY key, market;

Today we currently report the agg as partially ordered using market. For large inputs, this blocks releasing finished groups early

To Reproduce

EXPLAIN
WITH t(key, market) AS (
    VALUES
        (1, 'US'),
        (2, 'US'),
        (1, 'US')
)
SELECT key, market, COUNT(*)
FROM t
WHERE market = 'US'
GROUP BY key, market;

As of today, this produces

AggregateExec: mode=Partial,
  gby=[key@0 as key, market@1 as market],
  ordering_mode=PartiallySorted([1])

Every row has market = 'US', so this column never changes and cannot show that a group has finished.

Expected behavior

Ideally should ignore fixed grouping columns when deciding whether earlier groups are complete. For the example above, agg should not use market as a group completion boundary. If another grouping column is genuinely ordered, we should still use that column

Additional context

#24697 is somewhat related to this fix. It separates input ordering from the question of whether a group is complete. A fixed column can be considered ordered, but it should not be used as that a group is complete.