socks-proxy-agent: trim IPv6 proxy host brackets

[aouxwoux]

Summary

• normalize bracketed IPv6 literal hosts when parsing SOCKS proxy URLs
• add a constructor regression test for socks://[::1]:1080

Fixes #437

Verification

• pnpm --filter agent-base build
• pnpm --filter socks-proxy-agent test
• pnpm --filter socks-proxy-agent build
• pnpm --filter socks-proxy-agent lint
• git diff --cached --check

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Latest commit: ec6d282

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