fix(Chat): fix statically rendered custom icons

[cpsievert]

Follow up to #1853

Apparently statically rendered custom icons were never working as intended. Here's an example:

import faicons

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>
"""

ui.br()

chat = ui.Chat(id="chat")
chat.ui(
    messages=[welcome],
    icon_assistant=faicons.icon_svg("otter"),
)

Currently, you get the default robot icon, when you should get an otter in the startup message.

This got overlooked because ui.Chat()'s messages are dynamically rendered (and thus, are included in the server-side state), but .ui(messages) is static (and thus is not)

[gadenbuie]

Thanks!