url: pathToFileURL duplicates root slash across platforms and misses empty extended UNC servername #67091

Version

v22.16.0 (also reproduces on main)

Platform

Microsoft Windows NT 10.0.26100.0 x64 (and POSIX platforms when passing `{ windows: true }`)

Subsystem

url

What steps will reproduce the bug?

url.pathToFileURL() has two bugs in lib/internal/url.js:

1. Cross-platform root path trailing slash duplication (resolved[resolved.length - 1] !== path.sep):
   When restoring trailing slashes stripped by path.resolve(), pathToFileURL() compares the last character of resolved against the host platform's path.sep instead of the target separator (windows ? '\\' : '/'):

   // On Windows (where path.sep === '\\'):
   console.log(url.pathToFileURL('/', { windows: false }).href);
   // Prints: 'file:////' (4 slashes instead of 'file:///')
   
   // On POSIX / Linux / macOS (where path.sep === '/'):
   console.log(url.pathToFileURL('C:\\', { windows: true }).href);
   // Prints: 'file:///C://' (double trailing slash instead of 'file:///C:/')

2. Extended UNC path empty servername bypass (hostnameEndIndex === 2):
   For extended UNC paths (\\?\UNC\...), prefixLength is 8, but the empty servername check hardcodes if (hostnameEndIndex === 2) instead of if (hostnameEndIndex === prefixLength):

   console.log(url.pathToFileURL('\\\\?\\UNC\\\\share', { windows: true }).href);
   // Prints: 'file:///share' instead of throwing ERR_INVALID_ARG_VALUE ('Empty UNC servername')

How often does it reproduce? Is there a required condition?

Always (100% reproducible).

What is the expected behavior? Why is that the expected behavior?

• url.pathToFileURL('/', { windows: false }).href should return 'file:///' on all platforms (including Windows).
• url.pathToFileURL('C:\\', { windows: true }).href and url.pathToFileURL('C:/', { windows: true }).href should return 'file:///C:/' on all platforms (including POSIX).
• url.pathToFileURL('\\\\?\\UNC\\\\share', { windows: true }) should throw ERR_INVALID_ARG_VALUE (Empty UNC servername), matching url.pathToFileURL('\\\\\\share', { windows: true }).

What do you see instead?

• url.pathToFileURL('/', { windows: false }).href on Windows returns 'file:////'.
• url.pathToFileURL('C:\\', { windows: true }).href on POSIX returns 'file:///C://'.
• url.pathToFileURL('\\\\?\\UNC\\\\share', { windows: true }).href returns 'file:///share'.

Additional information

In lib/internal/url.js:

• Line 1707 checks if (hostnameEndIndex === 2) instead of if (hostnameEndIndex === prefixLength).
• Line 1722 checks resolved[resolved.length - 1] !== path.sep instead of resolved[resolved.length - 1] !== (windows ? '\\' : '/').