[Bugfix]: Fix structured output in multi-turn gpt-oss (#34454)

Signed-off-by: Ben Browning <bbrownin@redhat.com>
Co-authored-by: Cyrus Leung <cyrus.tl.leung@gmail.com>
Co-authored-by: Cyrus Leung <tlleungac@connect.ust.hk>

tests/entrypoints/openai/test_gptoss_structural_tags_integration.py

@@ -23,6 +23,7 @@ class TestGptOssStructuralTagsIntegration:
         """Create a mock tokenizer."""
         tokenizer = Mock()
         tokenizer.encode = Mock(return_value=[1, 2, 3, 4, 5])
+        tokenizer.vocab = {"<|end|>": 6}
         return tokenizer
 
     @pytest.fixture