Proves the patched nsProtocolProxyService end to end: the binary performs the
RFC1929 user/pass handshake with the configured socks_username/password and
relays the page through the proxy — something Playwright's own proxy= can't do,
and which test_proxy only unit-checks at the pref level.
Fully hermetic so it runs identically locally and on the GitHub runner: a local
SOCKS5 server (requires auth, records the creds it saw) + a local HTTP target,
with the localhost target forced through the proxy via allow_hijacking_localhost
+ no_proxies_on="". No external site, no secrets. 3/3 local.
