Refactor type guards in Agent class to use TypeIs instead of TypeGuard

[max-muoto]

In most cases, TypeIs is preferred over TypeGuard as TypeGuard fails to narrow in the negative case. A simple example is how isawaitable use to work, where the negative case would wouldn't narrow the object at all keeping it unioned it with Awaitable.

import inspect
from collections.abc import Awaitable
from typing import reveal_type

async def f(t: Awaitable[int] | int) -> None:
    if inspect.isawaitable(t):
        reveal_type(t)  # Awaitable[Any]
    else:
        reveal_type(t)  # Awaitable[int] | int