Reject unencoded unix socket URLs instead of silently hitting localhost

[afonsojanu]

Closes #1767.

If a http+unix:// (or https+unix://) URL's socket path still has literal slashes in it instead of being percent-encoded, the WHATWG URL parser can't separate the socket path from the request path at all. Everything after // just becomes the pathname and the hostname comes out empty, so options.socketPath ends up as an empty string.

Right now that falls all the way through to actually issuing an HTTP request against localhost with a null path, which is a pretty confusing thing to debug. You end up chasing an ECONNREFUSED against a host you never asked for instead of getting any hint that the URL itself was the problem.

I checked exactly what request options were being built for a URL like http+unix:///path/to/socket.sock/the/actual/url:

{ socketPath: '', host: 'localhost', port: 80, path: null, ... }

so the fix is to catch that at the point the socket path gets pulled out of url.hostname, before any of that gets used to build a request. When the hostname is empty in the +unix: branch, the request now fails immediately with a message pointing at the actual fix (percent-encode the slashes), routed through the same callback() path used for aborted requests, so it reaches whatever was passed to .end() the normal way.

Added a test alongside the existing unix socket tests covering the correctly-encoded case, using a deliberately unencoded path and asserting on the new error message. Ran the full test/node/unix-sockets.js file (still 5/5 including the two pre-existing http/https cases) and eslint -c .eslintrc src test clean (matches what CI runs).

One thing I noticed but didn't touch: the local xo-based pre-commit hook currently fails on this file for reasons unrelated to this change (missing node: protocol prefixes, a few prettier formatting nits, a couple of guard-for-in warnings elsewhere in the file) — looks like pre-existing drift between the xo config and the codebase rather than anything introduced here, so I left it alone rather than bundling an unrelated cleanup into this PR.