Fix runtime type extraction

Author: dandavison

temporalio/nexus/handler.py

@@ -323,11 +323,16 @@ def workflow_run_operation(
     def factory(service: S) -> WorkflowRunOperation[I, O, S]:
         return WorkflowRunOperation(service, start_method)
 
-    setattr(
-        factory,
-        "__nexus_operation__",
-        nexusrpc.handler.NexusOperationDefinition(name=start_method.__name__),
+    type_annotations = typing.get_type_hints(start_method)
+    [output_type] = typing.get_args(type_annotations.pop("return"))
+    [input_type, _] = type_annotations.values()
+
+    factory.__nexus_operation__ = nexusrpc.handler.NexusOperationDefinition(
+        name=start_method.__name__,
+        input_type=input_type,
+        output_type=output_type,
     )
+
     return factory