[Bug]: LLM.predict_and_call raises ValidationError without react_chat_formatter

Finding: In an external report (not reproduced by KFA), llama-index-core 0.14.25 on Python 3.12.3/Linux raises a pydantic ValidationError from LLM.predict_and_call and apredict_and_call when the LLM is not a function-calling model and no react_chat_formatter is passed. The error is 'Input should be a valid dictionary or instance of ReActChatFormatter' on the ReActAgent formatter field (https://github.com/run-llama/llama_index/issues/23445). Reported conditions: the reporter used a MockLLM subclass with is_function_calling_model=False and called predict_and_call(tools, user_msg=...) with a FunctionTool. The traceback shows failure inside ReActAgent construction in llm.py, before the agent runs. The reporter says FunctionCallingLLM.predict_and_call hands off to the same ReAct fallback when the model is not function-calling, so plain calls on such models cannot work (source: the issue). Reported cause (hypothesis, not maintainer-confirmed): the reporter says the methods pass formatter=kwargs.get('react_chat_formatter'), which sends None explicitly when the kwarg is absent. ReActAgent.formatter is declared with a default factory, so it rejects None and the default only applies if the key is omitted. The reporter also says the agent is built before the try block, so the error reaches the caller. These line references were not verified here. Reported workaround: the reporter's script shows that passing react_chat_formatter=ReActChatFormatter.from_defaults() explicitly gets past the ValidationError. This is source-backed on a mock LLM and not verified by KFA; it does not show end-to-end behavior with real models. Fix status: none confirmed. The reporter proposes building the ReActAgent kwargs as a dict and adding formatter, output_parser and tool_retriever only when supplied, in both sync and async methods, and offers a PR. No PR, fix commit or release was seen in the supplied data, and the issue was open with no comments. Limitations: this is a single fresh external report with no maintainer response, no independent reproduction, and no KFA reproduction. Treat the diagnosis as a plausible hypothesis until maintainers confirm it or a fix is released.