site-builder: ContactForm non-relay output byte-identical (honeypot whitespace fix) + test guard

Fix non-relay form output to match pre-change byte-for-byte by conditionally omitting the honeypot and fields lines when empty. Add backward-compat regex assertion to catch extra blank lines.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

craft/src/components/forms/ContactForm.tsx

@@ -447,9 +447,7 @@ ContactForm.craft = {
 
   return {
     html: `${marker}<form action="${actionAttr}" method="POST"${formStyle ? ` style="${formStyle}"` : ''}>
-  ${honeypot}
-  ${fieldsHtml}
-  <button type="submit"${btnStyle ? ` style="${btnStyle}"` : ''}>${esc(props.submitText || 'Send Message')}</button>
+${honeypot ? `  ${honeypot}\n` : ''}${fieldsHtml ? `  ${fieldsHtml}\n` : ''}  <button type="submit"${btnStyle ? ` style="${btnStyle}"` : ''}>${esc(props.submitText || 'Send Message')}</button>
 </form>`,
   };
 };