refreshHostname throws generic "Value not defined" when custom domain missing from Cloudflare

https://sonarly.com/issue/6662?type=bug

When a user checks DNS records for their custom domain, the server crashes with a generic "Value not defined" error because the hostname doesn't exist in Cloudflare's custom hostnames, despite being set in the workspace database.

Fix: The fix replaces the bare `assertIsDefinedOrThrow(publicDomainWithRecords)` call in `refreshHostname()` with one that passes a proper `DnsManagerException` as the second argument.

Previously, when `getHostnameWithRecords()` returned `undefined` (because the hostname wasn't found in Cloudflare), the assertion threw the default `Error("Value not defined")`. Now it throws a `DnsManagerException` with:
- Code `HOSTNAME_NOT_REGISTERED` (already defined in the exception enum)
- A user-friendly message `"Domain is not registered in Cloudflare"`

This matches the existing pattern used throughout the same service (e.g., `HOSTNAME_ALREADY_REGISTERED` in `registerHostname()`) and uses the already-imported `msg`, `DnsManagerException`, and `DnsManagerExceptionCode` symbols — no new imports or files needed.

```typescript file=packages/twenty-server/src/engine/core-modules/dns-manager/services/dns-manager.service.ts lines=141-150
    assertIsDefinedOrThrow(
      publicDomainWithRecords,
      new DnsManagerException(
        'Hostname not found in Cloudflare',
        DnsManagerExceptionCode.HOSTNAME_NOT_REGISTERED,
        {
          userFriendlyMessage: msg`Domain is not registered in Cloudflare`,
        },
      ),
    );
```

packages/twenty-server/src/engine/core-modules/dns-manager/services/dns-manager.service.ts

@@ -138,7 +138,16 @@ export class DnsManagerService {
       options,
     );
 
-    assertIsDefinedOrThrow(publicDomainWithRecords);
+    assertIsDefinedOrThrow(
+      publicDomainWithRecords,
+      new DnsManagerException(
+        'Hostname not found in Cloudflare',
+        DnsManagerExceptionCode.HOSTNAME_NOT_REGISTERED,
+        {
+          userFriendlyMessage: msg`Domain is not registered in Cloudflare`,
+        },
+      ),
+    );
 
     await this.cloudflareClient.customHostnames.edit(
       publicDomainWithRecords.id,