[Miniflare] Consider buffering all requests

This code fails in miniflare but works when deployed in a worker:

```ts
env.R2_BUCKET.put(key, request.body); // request.body is the `Request` from the worker handler.
```

It fails with `Provided readable stream must have a known length (request/response body or readable half of FixedLengthStream)`. I've seen workarounds here with `FixedLengthStream` but since this is working in production I don't think it should fail in miniflare. Happy to give more information if needed.

wrangler version: 3.1.1

Start by reproducing the Miniflare failure with the R2_BUCKET.put(key, request.body) example and trace Miniflare's request-body handling. Compare its known-length requirements with deployed Worker behavior; done means this request body is accepted locally without requiring a FixedLengthStream, with relevant coverage added if the existing test setup exposes this path.

Written by the indexing model from the issue text.