refactor: simplify grace hash join by removing unused params and extracting helpers

- Remove unused parameters: _output_schema, _build_keys, _schema, _left_schema, _right_schema, _metrics
- Extract execute_hash_join helper to deduplicate HashJoinExec construction in 3 places
- Extract sub_partition_batches helper to deduplicate build/probe repartitioning
- Extract PROBE_PROGRESS_MILESTONE_ROWS constant for magic number

Author: andygrove