fix: declare ActionController::API in the RBS stubs

The static and sqlite CI jobs failed at RBS validation: the generated
signature for TransmissionsController names ActionController::API as
its superclass, and sig/support/framework.rbs only declared
ActionController::Base. The local run before the push looked green
because the exit code check read the tail of a grep pipeline, not
rake itself; a fresh clone reproduced the failure immediately. The
stub now declares the API class, and rake passes in a clean clone
with its own exit code checked.

Author: cardmagic

sig/support/framework.rbs

@@ -21,6 +21,9 @@ end
 module ActionController
   class Base
   end
+
+  class API
+  end
 end
 
 module Rails