feat(files): fix expires_after API shape (#3604)

This was just quite incorrect. See source here:
https://platform.openai.com/docs/api-reference/files/create

llama_stack/strong_typing/schema.py

@@ -92,7 +92,12 @@ def get_class_property_docstrings(
     :returns: A dictionary mapping property names to descriptions.
     """
 
-    result = {}
+    result: Dict[str, str] = {}
+    # Only try to get MRO if data_type is actually a class
+    # Special types like Literal, Union, etc. don't have MRO
+    if not inspect.isclass(data_type):
+        return result
+
     for base in inspect.getmro(data_type):
         docstr = docstring.parse_type(base)
         for param in docstr.params.values():