Fix SetOf.update_children writing to wrong attribute

SetOf.update_children was writing to self.types (non-existent) instead
of self.type, causing inner type resolution to silently fail for Set
fields. This was a copy-paste error from TupleOf which uses the plural
self.types attribute. Added a length assertion to match the ListOf
pattern.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: pradhankukiran

mountaineer/client_builder/types.py

@@ -129,7 +129,8 @@ def children(self):
         return [self.type]
 
     def update_children(self, children):
-        self.types = tuple(children)
+        assert len(children) == 1
+        self.type = children[0]
 
 
 class LiteralOf(TypeDefinition):