Reuse dictionary value hashes across final aggregation input slices #26813

Related to #26780.

Partial hash aggregation can emit its materialized groups in multiple slices. Slicing a dictionary rewrites the keys but retains the full values array:

partial aggregate dictionary
        |
        +-- slice 1 --+
        +-- slice 2 --+--> FinalAggregate
        +-- slice 3 --+      `-- hashes the same values for every slice

DictionaryGroupValuesColumn already reuses some lookup state while the values Arc is unchanged, but final aggregation's outer group-ID hashing still calls create_hashes for every incoming slice. The work therefore remains O(slices x dictionary cardinality) before the rows are scattered through their keys.

Final aggregation should retain dictionary value hashes across input batches and recompute them only when the values-array identity or hash state changes. The state should live with the final aggregation stream so independent executions do not share mutable caches.

Completion criteria:

• slices sharing a values Arc hash the dictionary values once
• replacing the values Arc invalidates the cached hashes
• composite group keys, null keys/values, and multiple dictionary key types remain correct
• cache memory is bounded and accounted
• a PartialAggregate -> Repartition -> FinalAggregate test records the number of dictionary values hashed