fix: simplify __setattr__ guard — remove unreachable InstanceState branch

The isinstance(value, InstanceState) check was unreachable because
_build_configuration uses vars(self) directly, bypassing __setattr__.
Simplify to always reject assignments to state names via __setattr__.

Fixes 100% branch coverage.

Author: fgmacedo

statemachine/statemachine.py

@@ -227,12 +227,9 @@ def _processing_loop(self, caller_future: "Any | None" = None) -> Any:
     def __setattr__(self, name, value):
         # Fast path: internal/private attributes are never state IDs.
         if not name.startswith("_") and name in self.__class__.states_map:
-            if not isinstance(value, InstanceState):
-                raise StateMachineError(
-                    _("State overriding is not allowed. Trying to add '{}' to {}").format(
-                        value, name
-                    )
-                )
+            raise StateMachineError(
+                _("State overriding is not allowed. Trying to add '{}' to {}").format(value, name)
+            )
         super().__setattr__(name, value)
 
     def __repr__(self):