think: expose the admitted turn's requestId on TurnContext for beforeTurn

`beforePersist` (`PersistContext.requestId`), `onChatResponse` (`ChatResponseResult.requestId`) and `onChatError` (`ChatErrorContext.requestId`) all identify the chat request they run for, but `beforeTurn`'s `TurnContext` does not. Anything a subclass starts in `beforePersist` for a request (a trace span, a timer, a ledger entry) cannot be matched to the turn that later runs for it, short of inferring the link from the message list, which breaks as soon as two requests overlap in the pre-persist window.

The id is already in scope where Think builds the hook's argument: every admitted turn runs inside `admittedTurnContext` (`packages/think/src/think.ts`), and `_runInferenceLoop` assembles `ctx` right before `this.beforeTurn(ctx)`:

```ts
const ctx: TurnContext = {
system,
messages,
tools,
model,
continuation: input.continuation,
body: input.body
};
```

Proposed change: one field.

```ts
const ctx: TurnContext = {
// ...as today
body: input.body,
requestId: admittedTurnContext.getStore()?.requestId
};
```

```ts
export interface TurnContext {
// ...
/**
* The chat request this turn runs for — the same id `beforePersist`,
* `onChatResponse` and `onChatError` see. Undefined outside an admitted
* turn (entry points that bypass admission).
*/
requestId?: string;
}
```

Optional, so programmatic entry points that bypass admission stay valid and no existing subclass changes.

Context: we open an OpenTelemetry span per turn in `beforePersist` (it is the first thing a message pays for, and a refused charge is a turn that never ran) and need `beforeTurn` to adopt that request's span. We run exactly this as a patch on `@cloudflare/think@0.17.0` today. Happy to open the PR if the shape is acceptable.