Pare down python Chat features

[cpsievert]

This PR removes some features in Chat that predate chatlas and add a lot of complexity to the implementation without much benefit.

For reference, here is the list:

• Chat(messages=) was removed. Use chat.ui(messages=) instead.
• Chat(tokenizer=) was removed. This was only relevant for .messages(token_limits=[]) which was also removed.
• Several parameters were removed from .messages(). This reflects an overall change approach for maintaining the conversation history sent to the LLM -- Chat should no longer be responsible for maintaining this conversation state, and instead, another stateful object (perhaps the one provided by chatlas, Pydantic AI, LangChain, etc.) should be used to maintain this state. That said, .messages() can still be useful if you want a reflection of the message state on the client.
• The transform_user_input and transform_assistant_response decorators were removed. This change also reflects the new recommendation of using another stateful object to submit input and retain conversation history.
• .user_input()'s transform parameter was removed (as a consequence of the previous point).
• The already deprecated .set_user_message() method was officially removed (in favor of .update_user_input())

NOTE: Although these features are removed here, we should do a py-shiny release that deprecates these features first, then removes them full-stop when we move to re-export shinychat in shiny.

Follow up

• Create shiny PR to deprecate these features
• Possibly externalize the _chat_normalize.py logic into a more explicit API for transforming provider-specific completion objects into a ChatMessage/ChatMessageDict. That way .append_message() et al. can have a stronger message type.
• When we eventually go to re-export shinychat in shiny, remove the "3rd party" LLM dependencies from shiny's pyproject.toml

[Copilot]

Pull Request Overview

This PR pares down the Chat module by removing legacy features and simplifying the API. Key changes include the removal of deprecated decorators, test files for transform functionality, and legacy tokenizer/provider type support.

Reviewed Changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
pkg-py/tests/playwright/chat/transform_assistant_stream/*	Removal of tests related to the transform_assistant_response decorator.
pkg-py/tests/playwright/chat/transform_assistant/, pkg-py/tests/playwright/chat/transform/	Entirely removed tests and app code for legacy transform features.
pkg-py/tests/playwright/chat/shiny_output/, pkg-py/tests/playwright/chat/shiny_input/, pkg-py/tests/playwright/chat/input-suggestion/, pkg-py/tests/playwright/chat/icon/, pkg-py/tests/playwright/chat/basic/*	Updated Chat instantiation to use ui() parameters for messages.
pkg-py/src/shinychat/_chat_types.py	Removed the TransformedMessage data class.
pkg-py/src/shinychat/_chat_tokenizer.py	Removed legacy tokenizer-related support.
pkg-py/src/shinychat/_chat_provider_types.py	Removed provider types and conversion functions.

Comments suppressed due to low confidence (4)

pkg-py/tests/playwright/chat/basic/test_chat_basic.py:44

• Confirm that removing the default assistant message expectation aligns with the new Chat API behavior; update documentation if necessary.

-            {"content": initial_message, "role": "assistant"},

pkg-py/src/shinychat/_chat_provider_types.py:1

• The removal of provider types represents a breaking change; ensure downstream consumers are informed and documentation is updated.

import sys

pkg-py/src/shinychat/_chat_tokenizer.py:1

• Verify that no remaining parts of the codebase rely on the legacy tokenizer functionality now that this file has been removed.

from __future__ import annotations

pkg-py/tests/playwright/chat/icon/app.py:18

• [nitpick] Ensure that moving the messages parameter from the Chat initializer to the ui() method is clearly communicated in API documentation to avoid user confusion.

chat_default.ui(icon_assistant=None)