🐛 Anchor ALLOWED_FONT_REGEX in a4a head validation#40530
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handleLink gates ad-creative
<link rel=stylesheet>hrefs withALLOWED_FONT_REGEX.test(href), but the regex has no start/end anchors so test() succeeds on any href that merely contains an allowlisted provider, e.g.https://evil.example/x.css#https://fast.fonts.net/, which then survives head sanitization and gets preloaded from the host document. The validator that defines this allowlist (LINK_FONT_STYLESHEET in validator-main.protoascii) matches it with RE2::FullMatch against the whole value, so wrap the alternation in^(?:...)$to match that. Added a regression test for a foreign origin that embeds a provider substring.