Add message into css_classes to ChatMessage's markup

[ahuang11]

Builds upon #6405

Text wrapped in pn.Row/pn.Column without the grey background looks extremely out of place; this alleviates it by attaching "message" into css_classes, unless the user provides their own css_classes.

Before:

After:

Note, no need to pass css_classes=["message"]; simply pass parts (previously pn.pane.Markdown(parts, css_classes=["message"]))

import panel as pn
from openai import AsyncOpenAI

pn.extension()


async def callback(contents: str, user: str, instance: pn.chat.ChatInterface):
    response = await aclient.chat.completions.create(
        model="gpt-3.5-turbo",
        messages=[{"role": "user", "content": contents}],
        stream=True,
    )
    message = None
    parts = ""
    indicator = pn.indicators.Dial(name="Length")
    async for chunk in response:
        part = chunk.choices[0].delta.content
        if part:
            parts += part
            indicator.value = len(parts)
            layout = pn.Row(
                parts,
                indicator,
            )
            yield layout


aclient = AsyncOpenAI()
chat_interface = pn.chat.ChatInterface(callback=callback, callback_user="ChatGPT")
chat_interface.send(
    "Send a message to get a reply from ChatGPT!", user="System", respond=False
)
chat_interface.servable()

[codecov]

Codecov Report

Attention: Patch coverage is 7.40741% with 25 lines in your changes are missing coverage. Please review.

Project coverage is 22.22%. Comparing base (5578f98) to head (0de3b7c).
Report is 1175 commits behind head on main.

Files	Patch %	Lines
panel/chat/message.py	6.25%	15 Missing ⚠️
panel/tests/chat/test_message.py	9.09%	10 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #6407       +/-   ##
===========================================
- Coverage   82.75%   22.22%   -60.54%     
===========================================
  Files         239      312       +73     
  Lines       34507    46074    +11567     
===========================================
- Hits        28555    10238    -18317     
- Misses       5952    35836    +29884     

Flag	Coverage Δ
ui-tests	?
unitexamples-tests	22.22% <7.40%> (-51.64%)	⬇️

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