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

backend/src/cyclone/rebill/pipeline_a.py

@@ -19,6 +19,8 @@ from cyclone.rebill.reconcile import ReconcileOutcome, OutcomeCategory
 
 @dataclass(frozen=True)
 class RebillClaim:
+    """Pre-emission 837P shape for Pipeline A. The serializer adapter (Task 12) converts this to a ClaimOutput for serialize_837."""
+
     claim_id: str           # reuses original claim_submit_id
     member_id: str
     procedure: str
@@ -35,9 +37,10 @@ def build_pipeline_a_claims(
     carc_decisions: list[CarcDecision],
     cas_reasons_per_visit: list[tuple[str, ...]],
 ) -> list[RebillClaim]:
-    """One input list per visit; align by index."""
+    """Zip three parallel per-visit lists by index; drop PAID/NOT_IN_835 outcomes and EXCLUDED CARC decisions; emit one frequency-7 RebillClaim per survivor."""
     out: list[RebillClaim] = []
-    for vo, carc, reasons in zip(visit_outcomes, carc_decisions, cas_reasons_per_visit):
+    # strict=True: all three lists must be the same length; a mismatch is a contract bug, fail loud.
+    for vo, carc, reasons in zip(visit_outcomes, carc_decisions, cas_reasons_per_visit, strict=True):
         if vo.category not in (OutcomeCategory.DENIED, OutcomeCategory.PARTIAL):
             continue
         if carc == CarcDecision.EXCLUDED:
@@ -50,7 +53,7 @@ def build_pipeline_a_claims(
             charge=vo.visit.billed,
             frequency_code="7",
             needs_review=(carc == CarcDecision.REVIEW),
-            original_carc_reasons=tuple(reasons),
+            original_carc_reasons=reasons,
         ))
     return out
 
@@ -58,14 +61,16 @@ def build_pipeline_a_claims(
 def write_pipeline_a_files(
     claims: list[RebillClaim],
     out_dir: Path,
-    serialize_837_fn,  # injected: cyclone.parsers.serialize_837.serialize_837
+    serialize_837_fn,  # injected: callable[[RebillClaim], str] (orchestrator-supplied adapter)
     tpid: str,
 ) -> list[Path]:
     """Write one 837P per claim, using HCPF-spec filenames via build_outbound_filename.
 
-    `serialize_837_fn(claim)` returns the X12 byte string. The filename is
+    `serialize_837_fn(claim)` returns the X12 string. The 837P envelope is
+    pure ASCII so we write it as text. The filename is
     `cyclone.edi.filenames.build_outbound_filename(tpid, '837P')`.
     """
+    # Lazy: keep cyclone.rebill.pipeline_a importable without zoneinfo from filenames.
     from cyclone.edi.filenames import build_outbound_filename
 
     out_dir.mkdir(parents=True, exist_ok=True)
@@ -74,6 +79,6 @@ def write_pipeline_a_files(
         body = serialize_837_fn(c)
         fname = build_outbound_filename(tpid, "837P")
         path = out_dir / fname
-        path.write_bytes(body)
+        path.write_text(body, encoding="ascii")
         paths.append(path)
     return paths

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,
 )