Auto merge of #25719 - pshaughn:controlbeforeform, r=jdm

Make HTMLFormControlsCollection rooted at the form's root, not rooted at the form itself

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We'd been misreading the first line of https://html.spec.whatwg.org/#dom-form-elements. The collection needs to be rooted higher than the form itself, so it can contain form controls elsewhere in the document with a form= content attribute. It is, as far as I can tell, unspecified whether "rooted at the form's root" is meant to live-update to a new root if the form is moved to another tree, and I'm assuming it doesn't have to.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #25711

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- [X] There are tests for these changes

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