refactor(vertex_ai): remove dead code and align tests with production usage

- Remove unreachable snake_case parent_key checks in field transformation.
- Remove redundant parent_key argument from unit tests to match production invocation.

litellm/llms/vertex_ai/gemini/transformation.py

@@ -257,13 +257,11 @@ def _transform_part_to_httpx_format(part: dict, parent_key: Optional[str] = None
         # These are user-defined keys that should be preserved.
         should_preserve_keys = False
         if isinstance(v, dict):
-            if k == "args" and parent_key in ["functionCall", "function_call"]:
+            if k == "args" and parent_key == "functionCall":
                 should_preserve_keys = True
             elif k == "response" and parent_key in [
                 "functionResponse",
-                "function_response",
                 "toolResponse",
-                "tool_response",
             ]:
                 should_preserve_keys = True
             elif k == "properties":

tests/litellm/llms/vertex_ai/gemini/test_transformation.py

@@ -309,7 +309,7 @@ def test_vertex_transformation_field_casing():
             "args": {"security_risk": "high", "other_param": 123},
         }
     }
-    transformed = _transform_part_to_httpx_format(part, parent_key="functionCall")
+    transformed = _transform_part_to_httpx_format(part)
     assert "functionCall" in transformed
     assert "args" in transformed["functionCall"]
     # Keys in args should NOT be camelCased