Think: expose the turn's AbortSignal to beforeTurn #2309

Problem

Think.beforeTurn(ctx) may await I/O or a model call before streamText starts. If the user stops the turn during that wait, the hook has no way to observe the cancellation: TurnContext does not expose an AbortSignal. The turn can therefore remain in preparation until the awaited work finishes or times out.

The signal already exists in _prepareInferenceInvocation(input): the hook context is built without it, while the same input.signal is passed to streamText later. The public TurnContext type has no corresponding field.

Requested API

Expose the turn's signal to subclass beforeTurn hooks:

interface TurnContext {
  // existing fields...
  abortSignal?: AbortSignal;
}

const ctx: TurnContext = {
  // existing fields...
  abortSignal: input.signal,
};

Optional matches TurnInput.signal? and preserves callers without a signal. A subclass can pass it to abortable I/O, stop waiting promptly, and return from beforeTurn at a safe boundary. Some preparation writes should finish atomically, so exposing the signal is more useful than automatically racing and abandoning the entire hook. This request is for the in-process subclass hook; it does not require sending an AbortSignal through extension RPC snapshots.

Use case

We currently carry this one-field patch on @cloudflare/think@0.17.0. Our beforeTurn may wait for image-history previews (up to 8 seconds) or a history-summary model call (up to 30 seconds). Before the patch, Stop could not interrupt those waits. With the signal threaded through the waits, a local full-stack test using the real WebSocket cancel frame ended the two stopped turns in 7 ms and 29 ms, and both conversations handled a subsequent turn normally.

Related: #2240 also requests a field on TurnContext, but for request correlation rather than cancellation.