[whatwg/fetch] Clarify handling of request body streams during redirects (Issue #1871)

2400030206 created an issue (whatwg/fetch#1871)

### What is the issue with the Fetch Standard?

## Clarify handling of request body streams during redirects

### Description
The Fetch Standard currently lacks clear specification for how request body streams should be handled when encountering HTTP redirects (3xx status codes).

### Specification Section
- Section: 4.5 HTTP redirect fetch
- Link: https://fetch.spec.whatwg.org/#http-redirect-fetch

### Problem
When a request with a `ReadableStream` body encounters a redirect:
1. Should the stream be rewound/reset for the redirected request?
2. What happens if the stream has already been partially consumed?
3. How should implementations handle non-replayable streams?

### Expected Behavior
The specification should explicitly define:
- Whether streams are replayed for redirected requests
- Error handling for non-replayable streams
- Requirements for stream state preservation

### Browser Implementation Status
- Chrome: Currently replays streams for same-origin redirects
- Firefox: Fails with TypeError for cross-origin redirects with streams
- Safari: Inconsistent behavior based on stream type

### Impact
This ambiguity causes cross-browser compatibility issues for applications using streaming uploads with redirects.

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