[whatwg/fetch] Block port 0 (PR #1847)

Tests: https://github.com/web-platform-tests/wpt/pull/54227.

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- [x] At least two implementers are interested (and none opposed):
   * WebKit
   * Gecko (confirmed out-of-band)
- [x] [Tests](https://github.com/web-platform-tests/wpt) are written and can be reviewed and commented upon at:
   * https://github.com/web-platform-tests/wpt/pull/54227
- [x] [Implementation bugs](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) are filed:
   * Chromium: N/A
   * Gecko: N/A
   * WebKit: 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. -->

(See [WHATWG Working Mode: Changes](https://whatwg.org/working-mode#changes) for more details.)


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<a href="https://whatpr.org/fetch/1847.html" title="Last updated on Aug 11, 2025, 11:48 AM UTC (ce2ace2)">Preview</a> | <a href="https://whatpr.org/fetch/1847/90e1a1e...ce2ace2.html" title="Last updated on Aug 11, 2025, 11:48 AM UTC (ce2ace2)">Diff</a>
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/fetch/pull/1847

-- Commit Summary --

  * Block port 0

-- File Changes --

    M fetch.bs (1)

-- Patch Links --

https://github.com/whatwg/fetch/pull/1847.patch
https://github.com/whatwg/fetch/pull/1847.diff

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