Unfortunately the situation on Android is that other apps cannot do anything with WhatsApp, and there's fuck all you can do about it as a user.
I shouldn't need Google special-casing Gemini to allow LLMs to interact with my messages. I should be able to wire up Tasker to WhatsApp on one end, and to OpenAI or Anthropic models of my choice via API calls on the other end. Alas, Android is basically like iPhone now, just with more faux choice of vendors and less quality control.