Set tool annotations by decorating the tool function, not in `chat.register_tool()`

Tool annotations should be attached to the tool definition as closely as possible. It's currently awkward to create to a tool function that has annotations because those annotations need to be passed to chat.register_tool() at registration, which might be in a different place than where the tool is defined. (The tool could be in packaged code and the registration in user code.)

Instead of this

import faicons
from chatlas import ChatOpenAI


def get_weather_forecast(
    lat: float, lon: float, location_name: str
) -> ContentToolResult:
    """Get the weather forecast for a location."""
    # Mocked weather data for demonstration purposes
    return {"temperature_2m": 18, "condition": "Partly Cloudy"}


# Create chat client and register tool
chat_client = ChatOpenAI(model="gpt-4.1-nano")
chat_client.register_tool(
    get_weather_forecast,
    annotations={  # << THIS COULD HAPPEN FAR FROM TOOL DEL
        "title": "Weather Forecast",
        "icon": faicons.icon_svg("cloud"),
    },
)

Maybe this

import faicons
from chatlas import ChatOpenAI, add_tool_annotations

@add_tool_annotations(
  title="Weather Forecast",
  icon=faicons.icon_svg("cloud")
)
def get_weather_forecast(
    lat: float, lon: float, location_name: str
) -> ContentToolResult:
    """Get the weather forecast for a location."""
    # Mocked weather data for demonstration purposes
    return {"temperature_2m": 18, "condition": "Partly Cloudy"}


# Create chat client and register tool
chat_client = ChatOpenAI(model="gpt-4.1-nano")
chat_client.register_tool(get_weather_forecast)

Advantages

This would also resolve some of the underlying issue in #169 where known standard MCP Tool Annotation properties could be named arguments that are documented, but additional kwargs are just added as tool annotations.

Start by reading the implementation of chat.register_tool() and the existing tool annotation handling, then review the related concerns in issue #169. The change is complete when annotations can be declared on the tool function with a decorator and register_tool() can use the decorated function without separately supplied annotations.