Add IPv6 URL formatting support with square brackets in DCV checker

[msuliq]

Summary

• Add format_host_for_url static method to wrap IPv6 addresses in square brackets for valid URL formatting
• Update HTTP-based validation to use formatted host in URLs for both WEBSITE_CHANGE and ACME_HTTP_01 methods
• Add unit tests covering IPv6/IPv4/domain handling in URL construction

Test plan

• Verify format_host_for_url correctly wraps IPv6 addresses (including loopback, full form, zone ID)
• Verify IPv4 addresses and domain names remain unchanged
• Verify HTTP validation URLs contain properly bracketed IPv6 addresses
• Run full test suite (130 tests passing)

Issues

Resolves #58

[birgelee]

This looks good to me.