fix(node): allow IPv6-only addresses

[Sagit-chu]

Summary

• accept bare IPv6 literals when validating node addresses
• preserve existing IPv4, hostname, and host:port handling
• add regression coverage for IPv6-only nodes, including scoped IPv6 addresses

Root cause

net.SplitHostPort treats a bare IPv6 literal as a malformed host:port pair because it contains multiple colons. The validator now recognizes IP literals before attempting host:port parsing.

Validation

• go test ./internal/http/handler/... (322 passed)
• go test ./... (692 passed)
• git diff --check

Closes #515