IPv6 Address Validation

Summary

Add strict IPv6 address validation.

Current State

IPv6 addresses in brackets are parsed but not validated.

Requirements

• Validate IPv6 address format
• Support all IPv6 formats (full, compressed, IPv4-mapped)
• Validate zone ID format
• Return specific error for invalid IPv6

Validation Rules

• 8 groups of 4 hex digits, colon-separated
• :: compression (only once)
• IPv4-mapped format (::ffff:192.168.1.1)
• Zone ID after %25

Acceptance Criteria

• Accept valid IPv6 addresses
• Reject malformed addresses with clear errors
• Tests for edge cases