fix: use correct namecheap credentials file format

- Update credentials file to use 'certbot_dns_namecheap:dns_namecheap_username' format
- This is the required format for certbot-dns-namecheap plugin
- Previously used incorrect format that plugin couldn't parse

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: Leechael

custom-domain/dstack-ingress/scripts/dns_providers/namecheap.py

@@ -40,8 +40,9 @@ def setup_certbot_credentials(self) -> bool:
 
             cred_file = os.path.join(cred_dir, "namecheap.ini")
             with open(cred_file, "w") as f:
-                f.write(f"dns_namecheap_username = {self.username}\n")
-                f.write(f"dns_namecheap_api_key = {self.api_key}\n")
+                f.write(f"# Namecheap API credentials used by Certbot\n")
+                f.write(f"certbot_dns_namecheap:dns_namecheap_username = {self.username}\n")
+                f.write(f"certbot_dns_namecheap:dns_namecheap_api_key = {self.api_key}\n")
 
             os.chmod(cred_file, 0o600)
             return True