Fix sort enforcement for single-partition output

[xudong963]

Summary

• make EnforceSorting account for parent SinglePartition requirements when inserting a missing sort
• use SortExec + SortPreservingMergeExec when a child must be both ordered and single-partitioned
• add a regression test that runs SanityCheckPlan for ordered single-partition output requirements

Tests

• cargo fmt --all --check
• cargo check -p datafusion-physical-optimizer --lib
• cargo test -p datafusion --test core_integration output_requirement_adds_merge_after_partition_preserving_sort -- --nocapture
• cargo test -p datafusion --test core_integration physical_optimizer::enforce_sorting -- --nocapture