[whatwg/fetch] Replace request's "window" with "traversable for user prompts" (PR #1823)

After much discussion in #1820 and #1821, this seems to be more correct. Closes #1820. Closes #1821. Helps https://github.com/whatwg/html/pull/11250 specify browser-initiated navigations better.

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- [x] At least two implementers are interested (and none opposed):
   * This seems to match all browser behavior.
- [ ] [Tests](https://github.com/web-platform-tests/wpt) are written and can be reviewed and commented upon at:
   * Not really testable in WPT, due to https://github.com/web-platform-tests/wpt/issues/16019 I believe.
- [x] [Implementation bugs](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) are filed:
   * Chromium: N/A
   * Gecko: N/A, although maybe a bug to encourage the blanking-out behavior would be appreciated?
   * WebKit: N/A, although maybe a bug to encourage the blanking-out behavior would be appreciated?
   * Deno (not for CORS changes): N/A
- [x] [MDN issue](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) is filed: N/A
- [x] The top of this comment includes a [clear commit message](https://github.com/whatwg/meta/blob/main/COMMITTING.md) to use. <!-- If you created this PR from a single commit, Github copied its message. Otherwise, you need to add a commit message yourself. -->

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---

Marking as "do not merge yet" as we should prepare PRs for all of https://dontcallmedom.github.io/webdex/w.html#window%40%40request%40dfn . But I don't want to do that until we've confirmed this is the right path, since it's my third attempt :sweat_smile:


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<a href="https://whatpr.org/fetch/1823.html" title="Last updated on May 9, 2025, 6:15 AM UTC (cd1da21)">Preview</a> | <a href="https://whatpr.org/fetch/1823/5a96806...cd1da21.html" title="Last updated on May 9, 2025, 6:15 AM UTC (cd1da21)">Diff</a>
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/fetch/pull/1823

-- Commit Summary --

  * Replace request's "window" with "traversable for user prompts"

-- File Changes --

    M fetch.bs (131)

-- Patch Links --

https://github.com/whatwg/fetch/pull/1823.patch
https://github.com/whatwg/fetch/pull/1823.diff

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