Add benchmark for sort pushdown Inexact path (row group reorder)

[zhuqi-lucas]

Is your feature request related to a problem or challenge?

The existing sort_pushdown_sorted benchmark covers the Exact path (sort elimination, scan limit). However, the Inexact path optimizations — reverse scan (#19064) and row group reorder by statistics (#21580) — are not benchmarked.

Without an Inexact benchmark, we can't:

• Measure the performance impact of row group reorder
• Validate future improvements (dynamic RG pruning [Epic] Dynamic row group pruning using TopK threshold during parquet scan #21399, global reorder per @Dandandan's suggestion in feat: reorder row groups by statistics during sort pushdown #21580)
• Detect regressions in the Inexact code path

Describe the solution you'd like

Extend benchmarks/bench.sh and queries under benchmarks/queries/sort_pushdown/ to add Inexact scenarios:

1. Data: Generate a single large file with multiple row groups where row groups have overlapping or out-of-order statistics (forces Inexact path). Can be done by:

   • Writing data in non-sorted order with small max_row_group_size
   • Creating synthetic data with controlled row group boundaries

2. Queries (benchmarks/queries/sort_pushdown/q5.sql, q6.sql, ...):

   • SELECT * FROM t ORDER BY col ASC LIMIT 10 — TopK + RG reorder
   • SELECT * FROM t ORDER BY col DESC LIMIT 10 — TopK + reverse scan + RG reorder
   • SELECT * FROM t ORDER BY col ASC LIMIT 1000 — larger LIMIT
   • Wide-row variant: SELECT * with many columns to show row-level filter benefit

3. Baseline comparison: With/without datafusion.optimizer.enable_sort_pushdown to isolate the optimization's impact.

Additional context

• Parent: [EPIC] Sort pushdown / partially sorted scans #17348 (sort pushdown optimization)
• Establish the high level API for sort pushdown and the optimizer rule and support reverse files and row groups  #19064 — TopK with reverse row group scan
• feat: reorder row groups by statistics during sort pushdown #21580 — Row group reorder by statistics (needs this benchmark to show impact)
• [Epic] Dynamic row group pruning using TopK threshold during parquet scan #21399 — Dynamic row group pruning (would also benefit from this benchmark)