fix: put custom serializer block after builtin serializers

airflow/serialization/serde.py

@@ -141,18 +141,18 @@ def serialize(o: object, depth: int = 0) -> U | None:
         qn = "builtins.tuple"
         classname = qn
 
-    # custom serializers
-    dct = {
-        CLASSNAME: qn,
-        VERSION: getattr(cls, "__version__", DEFAULT_VERSION),
-    }
-
     # if there is a builtin serializer available use that
     if qn in _serializers:
         data, serialized_classname, version, is_serialized = _serializers[qn].serialize(o)
         if is_serialized:
             return encode(classname or serialized_classname, version, serialize(data, depth + 1))
 
+    # custom serializers
+    dct = {
+        CLASSNAME: qn,
+        VERSION: getattr(cls, "__version__", DEFAULT_VERSION),
+    }
+
     # object / class brings their own
     if hasattr(o, "serialize"):
         data = getattr(o, "serialize")()