Async thread-centric streaming surface for the v3 protocol.
AsyncThreadStream is an async context manager that owns a
ProtocolSseTransport for one thread, dispatches commands (run.start,
run.respond), exposes typed subscriptions over a single shared SSE
(subscribe, events), surfaces lifecycle state (interrupted,
interrupts) via an always-on lifecycle watcher SSE, and provides typed
projections (thread.values, thread.messages, thread.tool_calls,
thread.extensions).
Direct port of libs/sdk/src/client/stream/index.ts.