GeezerVison

Description

Windows paths and namespaces are case-insensitive. When using url.pathToFileURL(), extended UNC paths prefixed with lowercase \\?\unc\ or mixed-case \\?\Unc\ were not recognized as extended UNC because the implementation strictly checked for uppercase '\\\\?\\UNC\\'.

As a result:

• The path was parsed as a standard UNC path with prefix length 2, extracting "?" as the hostname.
• Ada / WHATWG URL parser stripped the invalid hostname to empty "", swallowing the server name as a directory in the pathname (file:///unc/server/share/file.txt).
• Passing the resulting URL to fileURLToPath() threw a fatal TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must be absolute.

Additionally, extended UNC paths with empty servernames (e.g. \\?\UNC\\share or \\?\unc\\share) bypassed validation because the empty servername check was hardcoded to hostnameEndIndex === 2 instead of checking against prefixLength.

This PR:

1. Updates pathToFileURL to match \\?\UNC\ case-insensitively using StringPrototypeToUpperCase.
2. Updates the empty servername check to hostnameEndIndex === prefixLength, properly throwing ERR_INVALID_ARG_VALUE for empty extended UNC servernames.
3. Adds test coverage for lowercase and mixed-case extended UNC paths, as well as empty extended UNC servernames and missing resource paths.

Refs: #54262
Refs: #56302

Checklist

• test passes
• tests and/or benchmarks are included
• commit message follows commit guidelines