Need a way to return a response right after when there's a new subscription request.

I'm using the Subscription feature to track the progress of a task.
But sometimes when the task is completed while a client is sending a request to subscribe progress, it can't receive any events from the server because on the server, the task is already completed and there would be no events anymore. Therefore, the client just stays on the incomplete state without knowing that the task is already completed.

If the server can send an immediate response to the client right after receiving a subscription request, it would be able to send a response that the task is already completed.

No file or test is named. Start at the subscription request handling described in the issue and trace how completed tasks are represented. Confirm the intended immediate-response behavior with maintainers, then add coverage for a subscription request that arrives after completion and verify the client receives the completed state.

Written by the indexing model from the issue text.