Fix: Use escaped keys in StateProxy serialization

Corrected the use of escaped keys in serializing StateProxy mutations. Replaced instance where unescaped key was used, ensuring consistency and correct handling of keys with dots.

src/streamsync/core.py

@@ -239,7 +239,7 @@ def get_mutations_as_dict(self) -> Dict[str, Any]:
             serialised_value = None
             if isinstance(value, StateProxy):
                 if value.initial_assignment:
-                    serialised_mutations[key] = serialised_value
+                    serialised_mutations[escaped_key] = serialised_value
                 value.initial_assignment = False
                 child_mutations = value.get_mutations_as_dict()
                 if child_mutations is None: