fix(url): normalizeHost with ipv6 hostname

[chimurai]

Hi,

Been using mockttp for a long time in http-proxy-middleware to mock the proxy target server.

Great work on this library 🙏

Recently I updated httpxy (dependency of http-proxy-middleware) with improved IPv6 support: unjs/httpxy#136

After updating httpxy some of existing IPv6 tests started to fail: https://github.com/chimurai/http-proxy-middleware/blob/ffbf5bd59712704c9df178d5df61d96062a11b22/test/e2e/ipv6.spec.ts

These tests are using mockttp with as target server.

I applied the fixes from this PR in this draft PR: chimurai/http-proxy-middleware#1234

Looks like it fixes the issue.

For full transparency, I used Copilot to help with analysis and creating this PR and the description below.

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Summary

Fix IPv6 host serialization when reconstructing absolute request URLs from parsed destination data.

Mockttp already parses bracketed IPv6 Host headers correctly, but when rebuilding host[:port] strings it was dropping the brackets. For requests like Host: [::1]:8000, that produced invalid absolute URLs such as http://::1:8000/api, which fail URL parsing and trigger a 400 before request handlers run.

Changes

• Update host normalization to re-add brackets when serializing IPv6 literal hostnames
• Preserve existing default-port behavior so :80 for HTTP and :443 for HTTPS are still omitted
• Leave parsing behavior unchanged for bracketed IPv6 input
• Add regression coverage for:
  • IPv6 normalization with non-default HTTP and HTTPS ports
  • IPv6 normalization with default ports
  • Request preprocessing from Host: [::1]:8000
  • Existing IPv4 and DNS host formatting behavior

Example

Before:

Host: [::1]:8000
reconstructed URL: http://::1:8000/api
URL parsing fails, request returns 400

After:

Host: [::1]:8000
reconstructed URL: http://[::1]:8000/api
request is processed normally

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