Consider first class support for Pydantic AI

I fully understand why y'all are developing [chatlas](https://github.com/posit-dev/chatlas). It's nice to have a posit-y GenAI middleware in Python that mostly mirrors y'all's `ellmer` R package. And I appreciate the tasteful coupling y'all have done between `chatlas` and `shinychat`. But I just felt obligated to open this issue to at least raise the idea of leaning into Pydantic AI instead. Either in addition to `chatlas` or as a full replacement. For the simplest use cases, it is pretty easy to swap `pydantic_ai` for `chatlas` and still leverage `shinychat`. But y'all have put in the (tasteful) bindings in here on `chatlas.ContentToolResult`. So anyone wishing to adopt `pydantic_ai` instead of `chatlas` will need to shim their own means of displaying tool calls. It might be nice if y'all centrally did the work of displaying `pydantic_ai` tool calls with `shinychat`. Some potential reasons to consider this: - Pydantic AI is getting a lot of investment and developer focus. Might as well piggy back that. - They're staying ahead of chatlas on a pile of things like A2A. - Pydantic AI has much tighter bindings to FastMCP. Chatlas does obviously work with FastMCP... but not to the same extent.