refactor(parquet sampling): address PR apache#22000 review feedback

Two changes responding to review on the parent commit:

1. Key sampling on a stable `file_index` instead of `file_name`
   (apache#22000 (comment)).

   Both `apply_row_group_sampling` and `apply_row_fraction_sampling`
   now take `file_index: usize` rather than `file_name: &str`. The
   parquet opener passes the execution `partition_index`. This makes
   sampling reproducible across environments (no dependency on the
   on-disk path), while still decorrelating files assigned to
   different partitions.

2. Extract the row-window selection into `build_row_window_selectors`
   and add fuzz coverage
   (apache#22000 (comment)).

   The previous inline arithmetic could produce overlapping windows
   when `target_rows` was close to `total_rows`: `window_size =
   ceil(target / n_windows)` could exceed `stride = total / n_windows`,
   so adjacent strides' windows would intersect. The extracted
   function caps `window_size` at `stride` (the construction that
   guarantees disjointness) and is covered by:

   * `row_window_selection_basic_layout` — hand-checked anchor case.
   * `row_window_selection_returns_none_on_invalid_input` — degenerate
     inputs return `None` cleanly.
   * `row_window_selection_full_target_no_overlap` — the previously
     buggy `target_rows == total_rows` case.
   * `row_window_selection_fuzz_invariants` — 5 000 randomized
     `(total_rows, target_rows, cluster_size, seed)` configurations,
     asserting full coverage, in-bounds positions, and no overlap.
   * `row_window_selection_fuzz_determinism` — 1 000 iterations
     verifying identical seeds produce identical layouts.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: adriangb

datafusion/datasource-parquet/src/opener.rs

@@ -896,17 +896,21 @@ impl FiltersPreparedParquetOpen {
         // Apply optional row-group and row-range sampling now that we
         // know the actual row-group count. Both calls are no-ops when
         // their respective fraction is `None`. Selection is
-        // deterministic per `(file_name, row_group_index, fraction,
-        // cluster_size)` so re-runs match.
+        // deterministic per `(partition_index, row_group_index,
+        // fraction, cluster_size)` so re-runs match. The execution
+        // `partition_index` is the stable per-file id we plumb in:
+        // it makes sampling reproducible across environments without
+        // depending on object-store paths, and decorrelates files
+        // assigned to different partitions.
         prepared.sampling.apply_row_group_sampling(
             &mut initial_plan,
             rg_metadata.len(),
-            &prepared.file_name,
+            prepared.partition_index,
         );
         prepared.sampling.apply_row_fraction_sampling(
             &mut initial_plan,
             rg_metadata,
-            &prepared.file_name,
+            prepared.partition_index,
         );
 
         let mut row_groups = RowGroupAccessPlanFilter::new(initial_plan);