fix(gateway): null-coalesce mode in SessionResetPolicy.from_dict

Complete the YAML null handling for all three SessionResetPolicy fields.
at_hour and idle_minutes already had null coalescing; mode was still
using data.get('mode', 'both') which returns None when the key exists
with an explicit null value.

Add regression test covering all-null input.

Based on PR #1120 by stablegenius49.

tests/gateway/test_config.py

@@ -83,6 +83,14 @@ class TestSessionResetPolicy:
         assert policy.at_hour == 4
         assert policy.idle_minutes == 1440
 
+    def test_from_dict_treats_null_values_as_defaults(self):
+        restored = SessionResetPolicy.from_dict(
+            {"mode": None, "at_hour": None, "idle_minutes": None}
+        )
+        assert restored.mode == "both"
+        assert restored.at_hour == 4
+        assert restored.idle_minutes == 1440
+
 
 class TestGatewayConfigRoundtrip:
     def test_full_roundtrip(self):