fix(domain): avoid duplicating railway-verify prefix

[onthebed]

Summary

• add a small formatter that only prepends railway-verify= when the token is bare
• reuse that formatter for railway domain DNS output and the matching MCP verification output
• add regression tests covering both prefixed and bare verification tokens

Closes #851

Testing

• cargo fmt --all -- --check
• cargo check
• cargo clippy --all-targets --all-features
• cargo test

[onthebed]

The only failing check on this PR is , which requires one of the release labels (, , , or ). I tried to add from my side, but GitHub permissions for this fork do not allow adding labels. If a maintainer can apply the appropriate release label, the PR should be unblocked on that check.