Add a GroupsAccumulator for array_agg with ORDER BY #26409

Is your feature request related to a problem or challenge?

array_agg(x ORDER BY y) has no GroupsAccumulator: groups_accumulator_supported requires order_bys to be empty, so every grouped ordered ARRAY_AGG goes through GroupsAccumulatorAdapter, with one OrderSensitiveArrayAggAccumulator per group. High-cardinality GROUP BY queries then pay the adapter's per-group allocation, per-group dispatch and ScalarValue output costs.

#26357 adds a native groups accumulator for array_agg(DISTINCT x), including ORDER BY x. The general ORDER BY y case is still on the adapter path.

Describe the solution you'd like

A native groups accumulator for ordered ARRAY_AGG, along the lines of #26357 and the ordered first_value/last_value groups accumulators:

• Store values and ordering keys in shared buffers for all groups, plus a (group_index, row_index) entry per value, instead of one accumulator per group.
• In evaluate, counting-sort the entries by group, then sort each group's slice by its ordering key.
• When is_input_pre_ordered is set (the input already satisfies the ordering), skip the per-group sort for raw input. States merged from several partitions still need sorting.

Describe alternatives you've considered

Keep the adapter. #25199 reduced the per-group memory of OrderSensitiveArrayAggAccumulator, but the per-group accumulator and dispatch remain.

Additional context

Follow-up to #26347 / #26357. I'd like to work on this.