feat(pkg-py): Improvements for statically rendered messages

[cpsievert]

This PR adds 2 things:

1. Chat.chat_ui(messages=...) now supports any type also supported by message_content()
2. ChatMessage() can now be constructed outside of a Shiny session

[Copilot]

Pull Request Overview

This PR improves support for statically rendered messages in the shinychat package by making two key changes: allowing ChatMessage() construction outside Shiny sessions and expanding message type support for Chat.chat_ui().

• Enhances ChatMessage() construction to work outside of active Shiny sessions by using get_current_session() instead of require_active_session()
• Updates Chat.chat_ui(messages=...) to accept any type supported by message_content() function
• Refactors multiple test apps to use the new chat.ui(messages=...) pattern instead of passing messages to the Chat() constructor

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.

File	Description
pkg-py/src/shinychat/_chat_types.py	Updates ChatMessage to handle session-less construction with fallback HTML processing
pkg-py/src/shinychat/_chat.py	Simplifies chat_ui() function to use message_content() for message processing
pkg-py/tests/playwright/chat/*/app.py	Refactors 5 test applications to use new chat.ui(messages=...) pattern
pkg-py/CHANGELOG.md	Documents the new features in the unreleased section

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