Updates to the ChatGenerator protocol

[sjrl]

Two ideas for updating the ChatGenerator protocol:

1. Update the return type of the run method to at least require replies: list[ChatMessage] since we rely on that key being present when we use the protocol in the codebase. This should be possible through using TypedDict, total=False and Required (still needs to be tested). So something like

from typing import Protocol
from typing_extensions import Required, TypedDict
from haystack.dataclasses import ChatMessage


class ChatGeneratorOutput(TypedDict, total=False):
    """
    Return type for Chat Generators.

    ``replies`` is always required. Implementations may include additional keys.
    """
    replies: Required[list[ChatMessage]]


class ChatGenerator(Protocol):
    def run(self, messages: list[ChatMessage]) -> ChatGeneratorOutput:

2. Consider adding a run_async to the protocol since we have finished adding run_async to all of our chat generators in core and in core-integrations Add async run method for ChatGenerators in haystack core #8858 and Add async run method for all ChatGenerators haystack-core-integrations#1379. Only a consideration since it's not guaranteed that community made components follow the protocol, but we are starting to add run_async methods to components that use the ChatGenerator protocol so I wonder if creating a second protocol (e.g. ChatGeneratorWithAsync) would be an alternative.

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