BUG: OpenAIResponseTarget passes non-object tool arguments to custom functions #3516

Describe the bug

In OpenAIResponseTarget._execute_call_section_async, a registered Tool gets a malformed_arguments result (or a ValueError with fail_on_missing_function=True) when the model sends arguments that parse as JSON but are not an object, such as null, [1, 2] or "text". Functions passed through custom_functions skip that check and are called with the raw None, list or string, so a function that reads args["x"] or args.get("x") raises and the whole send fails instead of the model getting an error it can recover from.

Steps to Reproduce

Register custom_functions={"lookup": fn} where fn does args.get("q"), and have the model return a function_call for lookup with "arguments": "null".

Expected Results

Same as a registered Tool: the model receives a malformed_arguments function output and the loop continues.

Actual Results

AttributeError: 'NoneType' object has no attribute 'get' is raised out of send_prompt_async.

Versions

Linux, Python 3.13.16, PyRIT 1.2.0.dev0 from main (96de757)