fix: DNS, add public suffix support, remove 2 underscores from txt (#4124)

## Description

- [x] - Todo Backward compatibility in SaaS

ref #2744

closes #3124

Corresponding SaaS PR must be merged on Release

## Steps for reproduction

<img width="979" alt="image"
src="https://github.com/user-attachments/assets/ac73026d-610a-42b5-a493-a37442501d76">

<img width="339" alt="image"
src="https://github.com/user-attachments/assets/4e0061e5-6c83-482d-8c23-6b76063d93e0">

https://new-txt.poke-method.work/

See test for public suffix.


## Code Review

- [ ] hi @kof, I need you to do
  - conceptual review (architecture, feature-correctness)
  - detailed review (read every line)
  - test it on preview

## Before requesting a review

- [ ] made a self-review
- [ ] added inline comments where things may be not obvious (the "why",
not "what")

## Before merging

- [ ] tested locally and on preview environment (preview dev login:
5de6)
- [ ] updated [test
cases](https://github.com/webstudio-is/webstudio/blob/main/apps/builder/docs/test-cases.md)
document
- [ ] added tests
- [ ] if any new env variables are added, added them to `.env` file

Author: istarkov

apps/builder/app/builder/features/topbar/cname.test.ts

@@ -0,0 +1,39 @@
+import { describe, test, expect } from "@jest/globals";
+
+import { extractCname } from "./cname";
+
+describe("extractCname from ordinary domains", () => {
+  test('should return "@" for top level like domains', () => {
+    expect(extractCname("example")).toBe("@");
+  });
+
+  test('should return "@" for root level domains', () => {
+    expect(extractCname("example.com")).toBe("@");
+  });
+
+  test("should return the subdomain", () => {
+    expect(extractCname("sub.example.com")).toBe("sub");
+  });
+
+  test("should return all subdomains", () => {
+    expect(extractCname("sub.sub.example.com")).toBe("sub.sub");
+  });
+});
+
+describe("extractCname from public suffix domains", () => {
+  test('should return "@"', () => {
+    expect(extractCname("co.za")).toBe("@");
+  });
+
+  test('should return "@"', () => {
+    expect(extractCname("example.co.za")).toBe("@");
+  });
+
+  test("should return all the subdomain", () => {
+    expect(extractCname("sub.example.co.za")).toBe("sub");
+  });
+
+  test("should handle domains with multiple public suffixes correctly", () => {
+    expect(extractCname("sub.sub.example.co.za")).toBe("sub.sub");
+  });
+});