test(bedrock): assert empty additionalModelRequestFields is omitted

The Converse transformer now drops an empty additionalModelRequestFields
block instead of sending it as `{}`. Update test_bedrock_top_k_param so
models without top_k support (llama3) assert the key is absent rather than
equal to an empty dict.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/llm_translation/test_bedrock_completion.py

@@ -2712,6 +2712,10 @@ def test_bedrock_top_k_param(model, expected_params):
         data = json.loads(mock_post.call_args.kwargs["data"])
         if "mistral" in model:
             assert data["top_k"] == 2
+        elif expected_params == {}:
+            # Models that don't support top_k produce no additionalModelRequestFields;
+            # the empty block is now omitted entirely rather than sent as `{}`.
+            assert "additionalModelRequestFields" not in data
         else:
             assert data["additionalModelRequestFields"] == expected_params