fix(sp41): tighten pipeline_a write_files contract + cleanup imports

backend/tests/test_rebill_pipeline_a.py

@@ -1,9 +1,7 @@
 """Pipeline A: denied/partial visits become frequency-7 replacement claims."""
 from datetime import date
 from decimal import Decimal
-from pathlib import Path
 from cyclone.rebill.pipeline_a import (
-    RebillClaim,
     build_pipeline_a_claims,
     write_pipeline_a_files,
 )