Collapse all leading slashes for special relative URLs

[MerlijnW70]

Summary

Special-scheme URLs with multiple leading slashes (e.g. ///test) were parsed
incorrectly and failed with EmptyHost. Resolving new URL("///test", "http://example.org/")
returned an error instead of http://test/, disagreeing with Chrome, Firefox,
Node and Bun.

Fix

When resolving a relative reference against a special base URL (http, https,
ws, wss, ftp), the WHATWG URL standard's special authority ignore slashes
state skips all leading slashes and backslashes before the authority.
parse_relative previously stripped only the initial // and parsed the
remainder as the authority, so ///test produced an authority of /test whose
leading slash yielded an empty host.

The fix branches on scheme_type.is_special(): a special URL now consistently
consumes every leading slash/backslash before the authority, while a non-special
URL keeps treating only the first // as the authority delimiter and the rest as
the path.

Test

No new fixtures were needed — the upstream WPT data in url/tests/urltestdata.json
already encodes the spec-mandated results. These seven ///-prefixed cases (base
http://example.org/) were removed from url/tests/expected_failures.txt and now
pass:

input	resolved href
///test	http://test/
///\//\//test	http://test/
///example.org/path	http://example.org/path
///example.org/../path	http://example.org/path
///example.org/../../	http://example.org/
///example.org/../path/../../	http://example.org/
///example.org/../path/../../path	http://example.org/path

The full url_wpt conformance suite passes with no other regressions (the harness
also flags "unexpected success", confirming these cases genuinely flipped from
failing to passing).

Reference

This is also the upstream cause of downstream issues such as
denoland/deno#35172, since Deno
parses URLs through this crate.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

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[Manishearth]

Hmm, can you help me understand how this matches the spec?

It seems like this only ignores a single additional slash since it consumes a slash, then goes into "ignore slashes" state, and then will validation error on the next slash.

[mrobinson]

It seems that this issue in Deno is attracting many duplicate LLM-generated pull requests.

[Manishearth]

If this is LLM generated we cannot accept it.

https://book.servo.org/contributing/getting-started.html#ai-contributions