fix(835): correct SVC04/SVC05 mapping per X12 005010X221A1

X12 835 SVC segment:
  SVC01 = composite procedure
  SVC02 = charge
  SVC03 = payment
  SVC04 = Unit or Basis for Measurement Code (UN, MJ, DA, ...)
  SVC05 = Service Unit Count

The parser previously read SVC04 as the units count and SVC05 as the
unit type — backwards. On real 835s (and the canonical minimal
fixture), SVC04 carries the code 'UN' which fails Decimal parsing, so
the units always came out as None and the code string was assigned to
unit_type. SP7's line-level matcher couldn't compare units on the SVC
side against the claim side because of this.

- _consume_service_payment: SVC04 → unit_type, SVC05 → units count
- Default unit_type to 'UN' when only the count is present
- minimal_835.txt + unbalanced_835.txt: swap positions to match spec
- Add 2 regression tests (units-and-unit-type, default-unit-type-to-UN)

backend/src/cyclone/parsers/parse_835.py

@@ -304,21 +304,20 @@ def _consume_service_payment(segments: list[list[str]], idx: int, line_no: int)
     payment = Decimal(seg[3]) if len(seg) > 3 and seg[3] else Decimal("0")
     units: Decimal | None = None
     unit_type: str | None = None
+    # X12 005010X221A1: SVC04 = Unit or Basis for Measurement Code
+    # (UN, MJ, DA, ...); SVC05 = Service Unit Count.
     if len(seg) > 4 and seg[4]:
-        # In X12 835, SVC04 is "units paid" and SVC05 is the unit-of-measure.
-        # SVC05 may carry a value (UN, MJ, etc.) or be empty.
+        unit_type = seg[4]
+    if len(seg) > 5 and seg[5]:
         try:
-            units = Decimal(seg[4])
+            units = Decimal(seg[5])
         except Exception:
             units = None
-    if len(seg) > 5 and seg[5]:
-        unit_type = seg[5]
-    elif len(seg) > 4 and seg[4]:
-        # Some payers omit SVC05 but still mean "UN" when the units value
-        # is present. Be conservative: only default to UN if no other value
-        # was set above.
-        if unit_type is None:
-            unit_type = "UN"
+    # Some payers omit SVC04 but still mean "UN" when SVC05 carries a
+    # count. Be conservative: only default to UN if no other value was
+    # set above.
+    if unit_type is None and units is not None:
+        unit_type = "UN"
     service_date: date | None = None
     ref_benefit_plan: str | None = None
     adjustments: list[ClaimAdjustment] = []