MCPService.serve(_:write:)

Returns once lines has ended *and* every request it carried has been answered. The end of input means no more requests, not abandon the ones in hand: a client that sent a batch and closed its end is still owed the replies, and cancelling them would drop answers that were already computed. Cancelling this call cancels active requests and queued output. The write closure must return cooperatively when its task is cancelled. AsyncStream concretely, rather than any non-throwing AsyncSequence: naming the failure type is what makes a sequence non-throwing, and that spelling needs macOS 15 where this package supports 13. Package-only transports share the generic request loop below.