[Bug] AsyncKiotaTransport does not cleanup resources correctly #494

class AsyncKiotaTransport(httpx.AsyncBaseTransport):
    """A custom transport that implements Kiota middleware functionality
    """

    def __init__(self, transport: httpx.AsyncBaseTransport, pipeline: MiddlewarePipeline) -> None:
        self.transport = transport
        self.pipeline = pipeline

    async def handle_async_request(self, request: httpx.Request) -> httpx.Response:
        if self.pipeline:
            response = await self.pipeline.send(request)
            return response

        response = await self.transport.handle_async_request(request)
        return response

 def _load_middleware_to_client(
        client: httpx.AsyncClient, middleware: Optional[list[BaseMiddleware]]
    ) -> httpx.AsyncClient:
        current_transport = client._transport
        client._transport = KiotaClientFactory._replace_transport_with_custom_kiota_transport(
            current_transport, middleware
        )
        if client._mounts:
            mounts: dict = {}
            for pattern, transport in client._mounts.items():
                if transport is None:
                    mounts[pattern] = None
                else:
                    mounts[pattern
                           ] = KiotaClientFactory._replace_transport_with_custom_kiota_transport(
                               transport, middleware
                           )
            client._mounts = dict(sorted(mounts.items()))
        return client