fix: resolve formatting, regression logic, and pytest defaults

- Run ruff format on all benchmark files (fixes CI lint job)
- Fix check_regression() substring ambiguity: ordered keyword matching
  so "latency_improvement_pct" is correctly classified as higher-is-better
- Update stale comments in conftest.py referencing wrong fixture
- Add pytest addopts to skip benchmark/slow/stress markers by default

tests/benchmarks/test_chromadb_stress.py

@@ -143,7 +143,10 @@ class TestBulkInsertPerformance:
             batch_end = min(batch_start + batch_size, n_docs)
             batch_docs = contents[batch_start:batch_end]
             batch_ids = [f"batch_{i}" for i in range(batch_start, batch_end)]
-            batch_metas = [{"wing": "test", "room": "bench", "chunk_index": i} for i in range(batch_start, batch_end)]
+            batch_metas = [
+                {"wing": "test", "room": "bench", "chunk_index": i}
+                for i in range(batch_start, batch_end)
+            ]
             col_batch.add(documents=batch_docs, ids=batch_ids, metadatas=batch_metas)
         batched_ms = (time.perf_counter() - start) * 1000