Fully sorted partial aggregate keeps per-batch accumulator scratch reserved after emitting completed groups (APPROX_DISTINCT) #26781

Describe the bug

#26726 (for #26725) releases the capacity that a clustered aggregate table keeps after it emits its completed groups under memory pressure, but only for the group values and group_indices. Accumulators can also keep per-batch scratch, and GroupsAccumulator has no way to release it.

HllGroupsAccumulator (APPROX_DISTINCT) keeps a hashes: Vec<u64> sized to the largest input batch. size() counts it, and state does not release it:

With 8192-row batches, each APPROX_DISTINCT keeps 64 KiB after the emit. As reported by @sunchao in #26726, a fully clustered partial aggregate with APPROX_DISTINCT(k) and APPROX_DISTINCT(v) over one sorted batch of 8192 distinct Int64 keys still fails a 100 KiB limit, although the table holds only the active group.

To Reproduce

A partial APPROX_DISTINCT(k), APPROX_DISTINCT(v) GROUP BY k over input sorted on k, with 8192 distinct keys per batch and a 100 KiB GreedyMemoryPool.

Expected behavior

After the memory-pressure emit, the reservation should cover only the active group's state, so the stream continues.

Additional context

One option: add a GroupsAccumulator::shrink_to_fit that does nothing by default, call it from ClusteredAggregateTable::shrink_to_fit, and implement it for accumulators that keep per-batch scratch, starting with HllGroupsAccumulator.

Willingness to contribute

I plan to contribute a fix for this bug independently