Auto merge of #25687 - pshaughn:taintfile, r=jdm

Filter file: and about: responses opaquely

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file: and about: schemes were being treated like data: for cors purposes, when in fact they should have been subject to opaque response filtering. A comment indicated that this was necessary for some CSS tests, but I think the comment was out of date; the only tests depending on the unfiltered responses are unit tests that do not test any CSS functionality. I've updated those tests to now test for the opaque-filtering itself.

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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 #25686

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- [X] There are tests for these changes, but also some loss of unit test coverage (#25693) and a newly failing test case (#25692)

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