Update summaries to use validate_has_extension with add_if_missing

Author: lossyrob

pystac/extensions/pointcloud.py

@@ -486,10 +486,7 @@ def ext(cls, obj: T, add_if_missing: bool = False) -> "PointcloudExtension[T]":
     def summaries(
         cls, obj: pystac.Collection, add_if_missing: bool = False
     ) -> "SummariesPointcloudExtension":
-        if not add_if_missing:
-            cls.validate_has_extension(obj)
-        else:
-            cls.add_to(obj)
+        cls.validate_has_extension(obj, add_if_missing)
         return SummariesPointcloudExtension(obj)