bench(csharp-query): tune scan auto prebuilt threshold

[s243a]

Summary

• Raises the scan benchmark auto-policy cutoff for small binary join workloads from 5,000 to 7,500 input rows.
• Names the cutoff as SmallJoinPrebuiltArtifactRowThreshold.
• Keeps non-join and n-ary join auto-policy behavior unchanged.

Verification

• git diff --check
• python3 examples/benchmark/benchmark_scan_materialization.py --scales 300 --modes join,nary_join --source-modes auto,preload,artifact-prebuilt --strategies auto --repetitions 1 --format report-tsv
• python3 examples/benchmark/benchmark_scan_materialization.py --scales 1k --modes join --source-modes auto,artifact- prebuilt,artifact --strategies auto --repetitions 1 --format report-tsv