Fix proxy-aware DNS pre-resolution for env proxies

[ryanfowler]

Summary

• Compute the effective explicit, environment, or system proxy before target DNS pre-resolution.
• Skip local DNS lookups when the selected proxy resolves the target remotely, and keep local resolution only for plain socks5://.
• Reuse the same proxy decision for redirect refresh handling and add a regression test for HTTPS_PROXY with --dns-server and -vvv.

Testing

• cargo fmt
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --all-features --test network env_https_proxy_skips_local_target_dns_preresolution
• cargo test --all-features --test network
• cargo test --all-features