Drain the current turn, checkpoint, stop the execution

Pauses an active session so it holds no worker: the running turn, if any, is drained, its checkpoint commits and the execution stops. Queued input is kept and runs after a resume. The session goes pausing and the receipt stays accepted until the execution is observed gone; then the session is paused and the receipt succeeded with result {resulting_admission_state, checkpoint_revision, queued_turn_count}. A session with no execution goes paused at once. A drain that takes over 60 seconds shows attention.code: PAUSE_BLOCKED with the reason; nothing is killed for it, and terminating is the caller's choice. The receipt fails if the pause cannot commit, a resume cancels it, or a terminate overtakes it. Each step is a status event with admission_state on the stream.

Scope: sessions:control (API key or cookie session). Idempotency-Key: required.