Studio: assert promoted fields on attribute path in test_extra_fields_accepted

This PR promoted frequency_penalty and seed from undeclared
chat-completion extras into explicit ChatCompletionRequest fields,
so they ride the attribute path now, not model_extra. The test
still asserted both via model_extra and failed on Linux Python
3.10-3.13 with 'assert None == 0.5'. response_format stays in
model_extra (still undeclared) so the extra='allow' contract is
covered by that branch.

studio/backend/tests/test_openai_tool_passthrough.py

@@ -275,18 +275,19 @@ class TestChatCompletionRequestToolFields:
         assert req.stop is None
 
     def test_extra_fields_accepted(self):
-        # `frequency_penalty`, `seed`, `response_format` are not yet
-        # explicitly declared but must survive Pydantic parsing now that
-        # extra="allow" is set.
+        # ``response_format`` is still an undeclared OpenAI-side field;
+        # it must survive Pydantic parsing because extra="allow" is set.
+        # ``frequency_penalty`` and ``seed`` were promoted to explicit
+        # ChatCompletionRequest fields in the sampling-params PR, so
+        # they now ride the attribute path, not model_extra.
         req = self._make(
             frequency_penalty = 0.5,
             seed = 42,
             response_format = {"type": "json_object"},
         )
-        # Extras land in model_extra
+        assert req.frequency_penalty == 0.5
+        assert req.seed == 42
         assert req.model_extra is not None
-        assert req.model_extra.get("frequency_penalty") == 0.5
-        assert req.model_extra.get("seed") == 42
         assert req.model_extra.get("response_format") == {"type": "json_object"}
 
     def test_unsloth_extensions_still_work(self):