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.

gateway/config.py

@@ -97,10 +97,11 @@ class SessionResetPolicy:
     @classmethod
     def from_dict(cls, data: Dict[str, Any]) -> "SessionResetPolicy":
         # Handle both missing keys and explicit null values (YAML null → None)
+        mode = data.get("mode")
         at_hour = data.get("at_hour")
         idle_minutes = data.get("idle_minutes")
         return cls(
-            mode=data.get("mode", "both"),
+            mode=mode if mode is not None else "both",
             at_hour=at_hour if at_hour is not None else 4,
             idle_minutes=idle_minutes if idle_minutes is not None else 1440,
         )