feat: globally sort shared FileStream work queue on Inexact sort pushdown

[adriangb]

Which issue does this PR close?

• Closes #.

Rationale for this change

After apache#21182 sorts files per-group and apache#21351 lets partitions share a single work queue via SharedWorkSource, the per-group order is lost at the queue level — each group's files land as a contiguous block regardless of cross-group stats. For Inexact pushdown (the common case for TopK DESC with a dynamic filter) we want workers pulling files in globally best-first order so the filter threshold tightens fastest across the whole scan.

Today's flow (Inexact, SharedWorkSource active):

G0 (sorted within): [lo_max, hi_max]      ┐
G1 (sorted within): [lo_max, hi_max]      ├─ flat_map → queue = [G0_lo, G0_hi, G1_lo, G1_hi]
...                                       ┘

Worker 0 gets G0_lo (whatever its max is — not necessarily the global best). TopK's dynamic filter tightens based on whatever max the first-completed scan happens to produce, not the global max.

What changes are included in this PR?

Carry a work_order_hint: Option<LexOrdering> on FileScanConfig — a desired-but-not-required ordering populated by the Inexact sort-pushdown branches (in rebuild_with_source and try_sort_file_groups_by_statistics). SharedWorkSource::from_config reads the hint and seeds the queue via a new sort_files_globally_by_statistics helper that flattens all files, builds one MinMaxStatistics, and orders by min value. Falls back to today's flat_map(FileGroup::iter) order when no hint is set or stats are unusable.

file_groups composition is untouched, so output_partitioning() stays stable — parallelism is preserved.

The hint is skipped on the reverse-scan path (reverse_file_groups = true): the per-group reversal must not be overridden by a globally ascending sort. A globally descending variant for that path is a possible follow-up.

Files changed:

• datafusion/datasource/src/file_scan_config/mod.rs — new work_order_hint field, threaded through builder, From<FileScanConfig> and build().
• datafusion/datasource/src/file_scan_config/sort_pushdown.rs — new sort_files_globally_by_statistics helper; hint populated in both Inexact branches.
• datafusion/datasource/src/file_stream/work_source.rs — SharedWorkSource::from_config consumes the hint; new unit tests for queue seeding.

Are these changes tested?

Yes:

• sort_pushdown_inexact_populates_work_order_hint — interleaved-stats multi-group Inexact scan; asserts the hint matches the requested ordering and partition count is preserved.
• sort_pushdown_rebuild_reverse_leaves_hint_none — reverse-scan path leaves work_order_hint = None.
• from_config_without_hint_preserves_flat_group_order — no-hint case matches today's queue order exactly.
• from_config_with_hint_seeds_globally_sorted_queue — queue is drained in global min-ascending order across groups, not group-concatenation order.
• from_config_falls_back_when_stats_missing — any file without stats falls back to flat order without panicking.

All 142 datafusion-datasource lib tests pass. Clippy clean on datafusion-datasource --all-targets --all-features.

Broader tests (full workspace clippy, physical-optimizer integration, sort_pushdown.slt) deferred to CI — local disk was exhausted by prior worktree build caches and the worktree target couldn't finish compiling the SLT binary. No changes outside datafusion-datasource so the blast radius is narrow.

Are there any user-facing changes?

No. work_order_hint is pub(crate) and purely an internal scheduling hint; no API changes. Existing behaviour is preserved when the hint is None.