Fix(Chat): Always ensure Anthropic user message comes first (not just when token_limits are provided)

[cpsievert]

Currently chat.messages(format="anthropic") only removes non-user starting messages if token_limits is not None (and we changed the default to None in #1657).

As a result, an example like this (where Chat(messages) are provided) now errors out after #1657:

from anthropic import AsyncAnthropic
from shiny.express import ui

ui.page_opts(
    title="Hello Shiny Chat",
    fillable=True,
    fillable_mobile=True,
)

chat = ui.Chat(id="chat", messages=["Hello! How can I help you today?"])
chat.ui()

llm = AsyncAnthropic()


@chat.on_user_submit
async def _():
    response = await llm.messages.create(
        model="claude-3-5-sonnet-20240620",
        messages=chat.messages(format="anthropic"),
        stream=True,
        max_tokens=1000,
    )
    await chat.append_message_stream(response)

This PR fixes the problem by having chat.messages(format="anthropic") always remove non-user starting messages.