feat: sync latest ARR implementation

monthly_reports/core.py

@@ -245,6 +245,11 @@ def build_monthly_report(
         raise MonthlyReportError(ErrorCode.SOURCE_INVALID, "monthly source rows are duplicated")
     for fact in facts:
         _validate_fact(fact, start, as_of_date)
+    if facts and max(fact.arrival for fact in facts) != as_of_date:
+        raise MonthlyReportError(
+            ErrorCode.SOURCE_INVALID,
+            "monthly as-of date must equal the greatest included ARRIVAL",
+        )
     pins = _validate_pins(facts, snapshot.daily_versions, start, as_of_date)
     _validate_observations(facts, snapshot.channel_observations, pins)
     ordered_channels = channel_order(snapshot)