Optimize hash operations and byte-wise row comparisons for dictionary encoding #21466

Two related performance improvements for dictionary-encoded columns during aggregation:

Byte-wise row comparisons : when checking for existing groups, group_rows.row(row) == group_values.row(*group_idx) compares raw bytes. Since dictionary encoding represents repeated values as integer indices, comparing those integers directly is significantly cheaper.
Hash operations : hash calls on dictionary-encoded columns currently operate on the full string/primitive values. Replacing these with hashes of the integer dictionary keys is a minor but meaningful performance gain.

Relevant Packages

Hash operations: datafusion-common
Byte-wise row comparisons: datafusion-physical-plan

Goals

1. Byte-wise row comparisons updated to compare dictionary integer indices instead of raw bytes
2. Hash calls on dictionary-encoded columns updated to use integer key values rather than decoded primitives
3. Performance benchmarks demonstrate measurable improvement
4. Regression tests for hashing confirm dictionary encoding changes don't violate hash guarantees

Note: Dictionary key→value mappings are not stable across RecordBatches;The same key index can map to a different value in a different batch. Any new logic must not assume otherwise.