Avoid __getitem__ calls on a PrefectFuture or State in checking…

… `Flow` parameter serialization (#14900)

src/prefect/flows.py

@@ -611,12 +611,17 @@ def serialize_parameters(self, parameters: Dict[str, Any]) -> Dict[str, Any]:
             # do not serialize the bound self object
             if self.ismethod and value is self.fn.__prefect_self__:
                 continue
+            if isinstance(value, (PrefectFuture, State)):
+                # Don't call jsonable_encoder() on a PrefectFuture or State to
+                # avoid triggering a __getitem__ call
+                serialized_parameters[key] = f"<{type(value).__name__}>"
+                continue
             try:
                 serialized_parameters[key] = jsonable_encoder(value)
             except (TypeError, ValueError):
                 logger.debug(
-                    f"Parameter {key!r} for flow {self.name!r} is of unserializable "
-                    f"type {type(value).__name__!r} and will not be stored "
+                    f"Parameter {key!r} for flow {self.name!r} is unserializable. "
+                    f"Type {type(value).__name__!r} and will not be stored "
                     "in the backend."
                 )
                 serialized_parameters[key] = f"<{type(value).__name__}>"