Return shape of useStream — the Angular StreamApi.
Reactivity primitives follow Angular conventions:
Signal<T>; call them as functions in
templates (stream.messages()). They are snapshots — never
mutate the returned arrays / maps.submit / stop / respond) are plain
functions. No WritableSignals are exposed on the root handle.client,
assistantId) are exposed as plain values; remount the
component to swap them.Promise that settles when the active thread's initial hydration
completes. Exposed so SSR/render-before-flush pipelines can
await stream.hydrationPromise before serialising.
v2 escape hatch — returns the bound ThreadStream.