feat(atc): enforce RFC 1123 TLD all-numeric rejection in IsHostName

[davidkallesen]

Summary

• Apply RFC 1123 §2.1 rule: top-level label must not be all-numeric
• Ensures IPv4 addresses (e.g. 192.168.0.27) are rejected by IsHostName
• Fixes ambiguity between hostnames and numeric IPv4-like strings

Changes

✨ Features

• Reject hostnames whose TLD is all-numeric per RFC 1123 §2.1
• Single-label all-numeric values (e.g. "123") are now rejected

🐛 Fixes

• IPv4 addresses no longer pass IsHostName (e.g. 192.168.0.27)
• "example.123" and similar numeric-TLD strings now return false

📝 Documentation

• Update IsHostName remarks to document the RFC 1123 TLD rule

🎨 Styling

• Expand IsHostName from expression-body to block for logic clarity