feat: implement retract_batch for array_agg(DISTINCT) sliding window

[SubhamSinghal]

Which issue does this PR close?

• Closes Add support for array_arg(DISTINCT x) in sliding window execution #22667.

Rationale for this change

What changes are included in this PR?

• DistinctArrayAggAccumulator state: HashSet → HashMap<ScalarValue, u64>.
• update_batch: increments the per-value count instead of inserting.
• New retract_batch: decrements, removes the key on zero, mirrors the update_batch null-handling
  rules (ignore_nulls skip, otherwise NULL is a tracked key).
• supports_retract_batch() now returns true.
• merge_batch is structurally unchanged — the wire state (List) carries presence, not
  multiplicities. Merged counts represent "partitions that emitted this value," which is fine because
  evaluate only reads keys. Refcount semantics are only relied on within a single accumulator instance
  (window execution, which doesn't merge).
• New helper ScalarValue::size_of_hashmap<V, S> in datafusion-common, mirroring size_of_hashset.

Are these changes tested?

Yes

Are there any user-facing changes?

Yes. array_agg(DISTINCT x) now works in bounded/sliding window frames. Queries that previously
errored now succeed: