Auto merge of #25236 - pshaughn:safelistct, r=jdm

De-deplicate is_cors_safelisted_request_header helper functions

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Separate is_cors_safelisted_request_header implementations in script::dom::request and net::fetch::methods have been merged to a single implementation in net_traits::request, with additional logic for spec requirements that weren't previously there. This doesn't pass all the failing tests, but it doesn't fail any passing ones either and it reduces confusion about what's supposed to happen where.

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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 #25235 and some but not all subcases in #25175

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- [X] There are tests for these changes, in that the WPT CORS tests that did already pass still do

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