fix(net): expose the SOCKS5 proxy-connect failure stage

[Antruly]

Problem

socks5_last_error() reports connection refused in two very different cases:

• the proxy TCP connect (or its DNS lookup / connect timeout) fails — the proxy was never reached;
• the proxy answers CONNECT with REP=0x05 — the proxy is reachable and the target refused.

Callers that classify failures (for example uvscope's proxy_unreachable vs target_rejected) currently cannot tell them apart from the public API.

Change

• Track the failure stage in uvcpp_tcp_client: set a flag on proxy connect / DNS / connect-timeout failures, reset it when a new SOCKS5 attempt starts.
• Export socks5_proxy_connect_failed() (read-only) and forward it from uvcpp_http_client.
• No behavior change for successful connects or for the existing error codes/messages.
• Add a functional test covering: dead proxy, proxy-rejected target, wrong password, and the successful path.

Verification

$ cmake --build build --target test_socks5_client_func
$ ./build/tests/functional/test_socks5_client_func
[socks5] connect_no_auth ok
[socks5] connect_domain_target ok
[socks5] connect_auth ok
[socks5] bind ok
[socks5] udp_associate ok
[socks5] proxy_failure_stage ok
[socks5] ALL PASSED

Also exercised by uvscope's smoke suite on v1.8.0 + this patch: dead proxy → proxy_unreachable, proxy REP=0x05 → target_rejected, wrong credentials → auth_failed.