fix: cap sort preserving merge statistics by fetch

[discord9]

Summary

• Cap SortPreservingMergeExec statistics by its fetch limit so top-k/scalar sides expose their small row count to physical optimizers
• Preserve no-limit statistics and avoid zeroing byte-size estimates when row count is unknown
• Add regression coverage for JoinSelection swapping an SPM(fetch=1) side to the hash join build side

Testing

• cargo test -p datafusion-common test_with_fetch
• cargo test -p datafusion-physical-plan sort_preserving_merge
• cargo test -p datafusion join_selection --test core_integration
• cargo check -p datafusion --test core_integration
• cargo fmt --all -- --check

Notes

• cargo clippy --all-targets --all-features -- -D warnings currently fails on unrelated existing 53-branch lints in datafusion/common/src/hash_utils.rs, pruning.rs, scalar/mod.rs, and error.rs; this patch does not touch those files.