fix: resolve ruff lint errors in benchmark suite

Remove unused imports (shutil, string, datetime, os, yaml, time,
SCALE_CONFIGS) and unused variable assignments in timing-only calls.

tests/benchmarks/data_generator.py

@@ -10,7 +10,6 @@ Planted "needle" drawers enable recall measurement without an LLM judge.
 import hashlib
 import os
 import random
-import string
 from datetime import datetime, timedelta
 from pathlib import Path