fix: return complete state (including transient keys) from deserialize()

- Change return from persistent_data to self._data
- Update docstring to reflect that complete state is returned

Co-authored-by: afarntrog <47332252+afarntrog@users.noreply.github.com>

Author: Copilot

src/strands/agent/state.py

@@ -165,13 +165,13 @@ def deserialize(self, data: bytes) -> dict[str, Any]:
             data: Serialized state bytes to restore
 
         Returns:
-            The deserialized state dictionary (excluding transient keys)
+            The complete state dictionary (including preserved transient keys)
         """
         persistent_data = self._serializer.deserialize(data)
         # Keep transient keys in memory, replace persistent
         transient_data = {k: v for k, v in self._data.items() if k in self._transient_keys}
         self._data = {**persistent_data, **transient_data}
-        return persistent_data
+        return self._data
 
     def _validate_key(self, key: str) -> None:
         """Validate that a key is valid.