Replace .get_latest_stream_result() with a more general .latest_stream

[cpsievert]

Follow up to #1846

Coming back to write about this, I changed my mind about get_latest_stream_result(). Having a more general .latest_stream is better, especially once I realized returning a "mock" ExtendedTask could get rid of some of the None/req(False) awkwardness.

Here's that same example from #1846 with the new API. It also adds status reporting and cancelling

import asyncio

from shiny import reactive
from shiny.express import input, render, ui

ui.input_action_button("cancel", "Cancel stream")

chat = ui.Chat("chat")

chat.ui()
chat.update_user_input(value="Press Enter to start the stream")


async def stream_generator():
    for i in range(10):
        await asyncio.sleep(0.25)
        yield f"Message {i} \n\n"


@chat.on_user_submit
async def _(message: str):
    await chat.append_message_stream(stream_generator())


@render.code
def _():
    return f"Status: {chat.latest_stream.status()}"

@render.code
async def stream_result_ui():
    return f"Result: {chat.latest_stream.result()}"


@reactive.effect
@reactive.event(input.cancel)
def _():
    chat.latest_stream.cancel()