fix(proxy): percent-decode username and password in parse_proxy()

[okxint]

Fixes #1761.

Problem

parse_proxy() uses urllib.parse.urlparse() to extract the userinfo
components, but urlparse returns the raw, still-percent-encoded strings
for username and password. A proxy URL like

socks5://alice:p%40ss@127.0.0.1:1080

(where the password is p@ss, percent-encoded because @ is a reserved
character in the authority component) produced:

>>> parse_proxy("socks5://alice:p%40ss@127.0.0.1:1080").password
'p%40ss'   # ← should be 'p@ss'

The encoded string p%40ss was then:

• base64-encoded verbatim by build_authorization_basic() for HTTP CONNECT
  proxies, producing a wrong Proxy-Authorization header (HTTP 407);
• passed unchanged to python-socks for SOCKS proxies, which rejected the
  connection because its own credential comparison expected the decoded form
  (ProxyError: failed to connect to SOCKS proxy).

This is a regression relative to python-socks itself (_helpers.parse_proxy_url
calls unquote() on both fields) and requests (same).

Fix

Apply urllib.parse.unquote() to username and password immediately after
urlparse(), so Proxy always stores plain (decoded) credentials.

Also remove the urllib.parse.quote() re-encoding in the IRI branch: that
block was encoding non-ASCII credentials back into percent-encoded form and
storing them in Proxy, which was equally wrong because both
build_authorization_basic() and python-socks expect decoded strings.

Tests

• Added two new entries to PROXIES_WITH_USER_INFO:
  • socks5://alice:p%40ss@proxy → ("alice", "p@ss")
  • socks5://alice%3Abob:secret%25word@proxy → ("alice:bob", "secret%word")
• Updated the existing non-ASCII IRI case to expect decoded credentials
  (("üser", "påss") instead of ("%C3%BCser", "p%C3%A5ss")).

[aaugustin]

Hello, could you clarify how this PR accounts for my spec of what needs to be done to solve the issue.

It seems that you've only solved a small subset. If everything else is actually OK, can you provide the evidence?

Please just don't throw an AI at my comment; I have an AI, and I'd rather run it myself than through you.