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/test_mcp_bench.py

@@ -14,7 +14,7 @@ import time
 import chromadb
 import pytest
 
-from tests.benchmarks.data_generator import PalaceDataGenerator, SCALE_CONFIGS
+from tests.benchmarks.data_generator import PalaceDataGenerator
 from tests.benchmarks.report import record_metric