fix: preserve fetch (LIMIT) when eliminating SortExec via Exact pushdown

When PushdownSort returns Exact and removes SortExec, the fetch (LIMIT)
from the original SortExec was lost. DataSourceExec then reads all data
instead of stopping early.

Now passes sort_exec.fetch() to inner.with_fetch() so DataSourceExec
gets limit=N and stops reading after N rows.

Benchmark impact (6M rows, reversed file names):
  Q2 ORDER BY LIMIT 100:      main 1237ms → 9ms (99% faster)
  Q4 SELECT * LIMIT 100:      main 5971ms → 24ms (99.6% faster)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: zhuqi-lucas

datafusion/physical-optimizer/src/pushdown_sort.rs

@@ -99,7 +99,14 @@ impl PhysicalOptimizerRule for PushdownSort {
             // Each node type defines its own pushdown behavior via try_pushdown_sort()
             match sort_input.try_pushdown_sort(required_ordering)? {
                 SortOrderPushdownResult::Exact { inner } => {
-                    // Data source guarantees perfect ordering - remove the Sort operator
+                    // Data source guarantees perfect ordering - remove the Sort operator.
+                    // Preserve the fetch (LIMIT) from the original SortExec so the
+                    // data source can stop reading early.
+                    let inner = if let Some(fetch) = sort_exec.fetch() {
+                        inner.with_fetch(Some(fetch)).unwrap_or(inner)
+                    } else {
+                        inner
+                    };
                     Ok(Transformed::yes(inner))
                 }
                 SortOrderPushdownResult::Inexact { inner } => {