Mark failing tests

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

Lib/test/test_float.py

@@ -727,6 +727,7 @@ class IEEEFormatTestCase(unittest.TestCase):
 
 class FormatTestCase(unittest.TestCase):
 
+    @unittest.expectedFailure  # TODO: RUSTPYTHON; ValueError: Invalid format specifier
     def test_format(self):
         # these should be rewritten to use both format(x, spec) and
         # x.__format__(spec)