Is your feature request related to a problem? Please describe.
Currently, to use the @tool decorator in Upsonic, users must create a class that inherits from Toolkit and implement the decorated methods within that class. This creates unnecessary boilerplate code for simple use cases where users just want to create a single tool function without the overhead of class creation.

For example, this simple case currently doesn't work:

from upsonic import Agent, Task
from upsonic.tools.decorators import tool


@tool(description="Fetch the stock of a product")
def fetch_warehouse_stock(product_name: str):
    return f"The stock of {product_name} is 100"


def main():
    web_agent = Agent(
        "Warehouse Stock Agent",
        model="gpt-4o-mini",  # type: ignore
    )

    task = Task(
        description="Who are you?",
        tools=[fetch_warehouse_stock],
    )

    result = web_agent.print_do(task)
    print(result)


if __name__ == "__main__":
    main()

Describe the solution you'd like
Enable support for standalone functions decorated with @tool to be used directly in the Task.tools list without requiring a Toolkit wrapper class. The system should automatically handle the conversion of decorated functions to the appropriate format expected by the Task validation.