Fix TLS negotiation for plain SOCKS proxies

[gilnatab (gilnatab)]

With Undici 7.29.0, patched fetch can't connect through plain SOCKS5 proxies. createProxyAgent always passes proxyTls, which tells Undici to use TLS for the proxy connection. It then sends a TLS handshake where the proxy expects a SOCKS5 handshake.

This PR passes proxyTls only for HTTPS proxies. It parses the URL first, so both https:// and HTTPS:// keep the proxy's CA settings. requestTls still handles TLS to the target server.

This also breaks SOCKS5 over TLS connections that worked before. Local tests of patched fetch with @vscode/proxy-agent 0.44.0 and Undici 7.29.0 gave these results for both HTTP and HTTPS targets:

Proxy endpoint	Version 0.44.0	This PR
Plain SOCKS5	Fails	Works
SOCKS5 over TLS with a trusted certificate	Works	Fails

If anyone relies on SOCKS5 over TLS through patched fetch, their requests would stop working after this change. We don't know whether anyone currently uses this setup.

Before merging, please decide whether we need to add a library option and a VS Code setting for SOCKS5 over TLS now, or whether they can come later. This PR doesn't add them. Users would need both to turn TLS back on through VS Code settings.

The tests cover plain SOCKS5 handshakes and lowercase/uppercase HTTPS proxy URLs. The code compiles, and unit tests pass on Node 22.22.2.

Related issue: openai/codex#35924. Local tests also reproduced this connection problem.