Standardize conversation agents passing in ChatSession

[balloob]

Context

Conversation agents have a new shared history class called ChatSession. Instead of requiring all conversation agents to request it, we can standardize a new method, in a backwards compatible way to pass this in.

Old method to process an incoming chat from a user:

    async def async_process(
        self, user_input: conversation.ConversationInput
    ) -> conversation.ConversationResult:
        """Process a sentence."""

Proposal

The proposal is to change it to the following:

    async def async_process(
        self, user_input: conversation.ConversationInput
    ) -> conversation.ConversationResult:
        """Process a sentence."""
        async with conversation.async_get_chat_session(
            self.hass, user_input
        ) as session:
            return await self._async_call_api(user_input, session)

    async def _async_handle_message(
        self,
        user_input: conversation.ConversationInput,
        # NativeT will be a generic type for the ConversationEntity
        session: conversation.ChatSession[NativeT],
    ) -> conversation.ConversationResult:
        """Call the API."""
        raise NotImplementedError

Conversation agents that leverage ChatSession can migrate their code to use the new _async_handle_message method. Unmigrated conversation agents can continue to use the async_process method as before.

Alternatives

We can inspect the async_process method to see if it supports the ChatSession parameter. We can also add a variable to the class to indicate if it implements v1 or v2 of the process API.

[allenporter]

Generally: Looks good to me. This seems like a good simple step forward as we expand the capabilities of ChatSession.

Do we want ConversationInput as a parameter of ChatSession? There are cases where I think the ChatSession needs arguments such as agent_id Notably the conversation_id in the ConversationInput is stale since the ChatSession may make a new id.