fix: avoid iterating invalid margin schedules

src/ml4t/backtest/config.py

@@ -288,6 +288,9 @@ def _coerce_margin_schedule(
         return None
     coerced: dict[str, Any] = {}
     for asset, value in schedule.items():
+        if not isinstance(value, list | tuple):
+            coerced[asset] = value
+            continue
         try:
             initial, maintenance = value
             coerced[asset] = (float(initial), float(maintenance))
@@ -303,6 +306,9 @@ def _margin_schedule_to_dict(
         return None
     serialized: dict[str, Any] = {}
     for asset, value in schedule.items():
+        if not isinstance(value, list | tuple):
+            serialized[asset] = _serialize_invalid_margin_scalar(value)
+            continue
         try:
             initial, maintenance = value
             serialized[asset] = [float(initial), float(maintenance)]
@@ -331,6 +337,11 @@ def _validate_margin_pct_schedule(
     if schedule is None:
         return issues
     for asset, value in schedule.items():
+        if not isinstance(value, list | tuple):
+            issues.append(
+                f"margin_pct_schedule[{asset!r}] must be a two-item (initial, maintenance) sequence"
+            )
+            continue
         try:
             initial, maintenance = value
         except (TypeError, ValueError):

tests/test_config_wiring.py

@@ -1021,24 +1021,33 @@ def test_invalid_margin_pct_schedule_export_does_not_raise(self):
         assert data["account"]["margin_pct_schedule"] == {"ES": [0.05]}
 
     def test_invalid_margin_pct_schedule_export_keeps_non_sequences_scalar(self):
-        def values():
-            yield "initial"
-            yield "maintenance"
-            yield "extra"
+        class CountingIterator:
+            def __init__(self):
+                self.read_count = 0
+
+            def __iter__(self):
+                return self
+
+            def __next__(self):
+                self.read_count += 1
+                return f"value-{self.read_count}"
+
+        iterator = CountingIterator()
 
         config = BacktestConfig(
             margin_pct_schedule={
                 "DICT": {"initial": 0.05, "maintenance": 0.035},  # type: ignore[dict-item]
-                "GEN": values(),  # type: ignore[dict-item]
+                "ITER": iterator,  # type: ignore[dict-item]
                 "STR": "im",  # type: ignore[dict-item]
             }
         )
 
         data = config.to_dict()
 
         assert isinstance(data["account"]["margin_pct_schedule"]["DICT"], str)
-        assert isinstance(data["account"]["margin_pct_schedule"]["GEN"], str)
+        assert isinstance(data["account"]["margin_pct_schedule"]["ITER"], str)
         assert data["account"]["margin_pct_schedule"]["STR"] == "im"
+        assert iterator.read_count == 0
 
     def test_margin_pct_schedule_float_overflow_is_reported(self, tmp_path):
         class OverflowFloat: