Cap SortPreservingMerge statistics by fetch

[discord9]

Which issue does this PR close?

• Closes none.

Rationale for this change

SortPreservingMergeExec supports fetch, but its statistics currently pass through child statistics unchanged. This can make cost-based optimizers overestimate top-k merge outputs. For example, a SortPreservingMergeExec(fetch=1) side can still look as large as its input to JoinSelection, causing hash join build/probe choices to miss the small top-k side.

Other fetch-aware operators such as SortExec, CoalescePartitionsExec, and limit execs already cap their statistics by fetch. This PR aligns SortPreservingMergeExec with that behavior.

What changes are included in this PR?

• Cap SortPreservingMergeExec statistics with Statistics::with_fetch(self.fetch, 0, 1).
• Preserve no-op statistics for fetch = None and skip = 0.
• Avoid scaling byte-size estimates by 0.0 when input row count is unknown; use absent byte-size stats instead.
• Add regression tests for SPM fetch statistics and for JoinSelection swapping an SPM(fetch=1) side to the hash join build side.

Are these changes tested?

Yes.

• 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
• cargo clippy --all-targets --all-features -- -D warnings

Are there any user-facing changes?

No user-facing API changes. This improves optimizer statistics for fetch-limited sort-preserving merge plans.