Clustered aggregation: release retained capacity before failing or spilling when no group is complete #26911

Is your feature request related to a problem or challenge?

When a clustered aggregate stream (partial, final or single, with full group clustering) fails to grow its reservation, it first emits the completed groups and then calls ClusteredAggregateTable::shrink_to_fit (#26726, #26782). If no group is complete, for example when one key spans the whole batch, the stream returns ResourcesExhausted (partial), or spills or errors (final/single) without shrinking. Shrinking alone could make the reservation fit, because the table still holds capacity it grew for the batch, such as the per-batch accumulator scratch.

For example, a partial APPROX_DISTINCT(v) GROUP BY k over three 8192-row batches of a single key fails a 64 KiB limit asking for 132.1 KB, although the table holds about 4.3 KB after shrink_to_fit.

Raised by @comphead in #26782.

Describe the solution you'd like

On a refused reservation with no completed group, call shrink_to_fit and retry the reservation before returning the error or spilling. The existing memory_pressure_without_completed_groups tests in the final and single streams would need to reflect this.

Describe alternatives you've considered

Bound the per-batch scratch itself, for example by hashing in fixed-size chunks in HllGroupsAccumulator, which also avoids regrowing the scratch for every batch under sustained pressure.