fix(proxy): Prevent duplicate domains and restore vhost on failure

[nfebe]

• Add duplicate domain+path validation in addDomain API endpoint
• Add nginx warning tolerance (ssl_stapling warnings no longer fail)
• Add location deduplication in config generator to prevent duplicate location "/" errors
• Add vhost backup/restore on proxy setup failure instead of deletion
• Add WriteVirtualHost and GetVirtualHost to NginxManager interface

Fixes issue where adding domains would fail due to ssl_stapling warnings and corrupt vhost config by deleting it on failure.

[sourceant]

Code Review Summary

✨ This pull request significantly enhances the robustness and reliability of the Nginx proxy configuration management within the agent. Key improvements include a new rollback mechanism for Nginx virtual host updates, preventing system outages from bad configurations. Additionally, the system now correctly handles duplicate domain definitions and location blocks, and Nginx config validation has been made more intelligent to distinguish between fatal errors and warnings.

🚀 Key Improvements

• Implemented a critical rollback mechanism for Nginx virtual host configuration changes in internal/proxy/orchestrator.go, ensuring previous stable configurations can be restored upon validation failure.
• Introduced deduplication logic for Nginx location blocks in internal/nginx/manager.go, preventing Nginx configuration syntax errors when multiple domain rules target the same path.
• Enhanced TestConfig in internal/nginx/manager.go to correctly identify and log Nginx configuration warnings without treating them as fatal errors, improving operational insights.
• Added validation in internal/api/server.go to prevent the creation of duplicate domains (based on domain name and path prefix), maintaining data integrity and preventing Nginx configuration conflicts.
• Expanded test coverage in internal/nginx/manager_test.go and internal/proxy/orchestrator_test.go to validate these new features and improvements.

[sourceant]

Review complete. See the overview comment for a summary.