fix(Chat, MarkdownStream): prevent escaping of HTML inside message content strings

[cpsievert]

Follow up to #1868. That PR introduced a problem where HTML content included in a starting message no longer rendered as HTML since the HTML started getting escaped. For a reprex, this example:

from shiny.express import ui

welcome = """
**Hello!** How can I help you today?

Here are a couple suggestions:

* <span class="suggestion">Tell me a joke</span>
* <span class="suggestion submit">Tell me a story</span>
"""

chat = ui.Chat(id="chat")
chat.ui(messages=[welcome])


@chat.on_user_submit
async def _(user_input: str):
    await chat.append_message(f"You said: {user_input}")

Went from:

to:

Note that:

1. This isn't a problem for append_message() et al. since we're already skipping the "tagify" step there for strings.
2. We have the same problem in shinychat, so I'll be doing a follow up PR there.