StreamingClaudeSession now buffers a mid-turn user message in a FIFO queue and
flushes one when the turn's result arrives (no implicit interrupt). InterruptAsync
only writes the control_request (no-op when idle); the resulting turn-end then
flushes any queued message. New InteractiveSessionService.InterruptAsync +
WorkerHub.InterruptInteractiveSession + IWorkerClient.InterruptInteractiveSessionAsync.