`controller.byobRequest` type incorrectly says value is possibly null while `autoAllocateChunkSize` is present.

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### 🔎 Search Terms

autoAllocateChunkSize

### 🕗 Version & Regression Information

- This is the behaviour in every version I tried, and I reviewed the FAQ for entries about readable, streams, byob, and autoallocatechunksize

### ⏯ Playground Link

https://www.typescriptlang.org/play/?ts=6.0.0-dev.20250806#code/HYUw7gBASiCGAmsBGAbEBlALgJzgWwAoBvAKAgkwE8AHEALggCIlLMQBnRgGjIlgFdMAewCCKFEIDGsNgGEAFv2ABrdAEsAXvQgBGAAwAmACw9y1fuIKShwHEPEhsASgily5a7ez202AHSSEuwgBE4A3LweNnYO-ixCSDAAjvwcmH647NQ28AR64bwAvjyFBUA

### 💻 Code

```ts
new ReadableStream({
type: "bytes",
autoAllocateChunkSize: 1024,
pull(controller) {
controller.close();
controller.byobRequest.respond(0);
},
});
```

### 🙁 Actual behavior

TypeScript says `controller.byobRequest` can be `ReadableStreamBYOBRequest | null` and errors when one tries to do `.respond` on it as `null` doesn't have that.

### 🙂 Expected behavior

TypeScript should say `controller.byobRequest` is `ReadableStreamBYOBRequest` because the `autoAllocateChunkSize` property is set. Meaning it can't be null according to https://developer.mozilla.org/en-US/docs/Web/API/ReadableStream/ReadableStream#autoallocatechunksize

### Additional information about the issue

_No response_

Start with the provided TypeScript Playground link and inspect the ReadableStream constructor typing around `autoAllocateChunkSize` and `controller.byobRequest`. Reproduce the nullability error, then determine how the typing should reflect the configured byte stream and verify that the example accepts `.respond(0)` without a null error.

Written by the indexing model from the issue text.