fix: lastResult should reflect resolved value

[gwansikk]

🎯 Changes

lastResult in AsyncDebouncerState, AsyncThrottlerState, and AsyncRateLimiterState is typed as ReturnType<TFn> | undefined, which resolves to Promise<T> | undefined since TFn extends AnyAsyncFunction. At runtime, the stored value is always the awaited result.

- lastResult: ReturnType<TFn> | undefined
+ lastResult: Awaited<ReturnType<TFn>> | undefined

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested this code locally with pnpm run test:pr.

🚀 Release Impact

• This change affects published code, and I have generated a changeset.
• This change is docs/CI/dev-only (no release).

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