fix(proxy): improve DNS handling and CONNECT tunneling

[ithewei]

Summary

• resolve fixed TCP/UDP proxy example targets before starting their event loops
• resolve HttpHandler hostname targets asynchronously through EventLoop::resolveDns, preserving the numeric-IP fast path and TLS SNI
• retry dropped DNS queries and keep DNS plus TCP connection setup within one proxy_connect_timeout budget
• add CONNECT tunneling support to tinyproxyd, including authority parsing and bounds checks
• fix [bug] use-after-free: hloop_free() on a loop with an in-flight SOCKS5 server DNS query #891 by keeping the DNS resolver alive through IO close callbacks, with a teardown regression test

Scope

The fixed-upstream library functions hloop_create_tcp_proxy_server and hloop_create_udp_proxy_server are unchanged; the example programs resolve their fixed targets before starting the loop. tinyproxyd remains a demo and still uses its existing socket creation path for ordinary dynamic targets.

Verification

• Debug CMake full build with unit tests, examples, Lua, MQTT, and KCP enabled
• hdns_test with MallocScribble=1 and MallocErrorAbort=1
• lua_io_test, lua_http_test, and lua_mqtt_test teardown coverage
• DNS simulator dropping the first query: proxy succeeds on retry (200 in about 0.54s)
• non-responsive DNS: proxy returns 504 in about 1.51s while /ping on the same worker remains responsive
• fixed TCP proxy round trip with a localhost target
• fixed UDP proxy echo with a localhost target
• SOCKS5 hostname round trip
• HttpHandler forward-proxy hostname round trip
• tinyproxyd CONNECT tunnels with numeric-IP and hostname authorities
• tinyproxyd ordinary HTTP proxy regression
• git diff --check

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.