Debian-LSP: Improve File Format Support

The Debian LSP Language Server currently provides only basic features — field completion, parse-error diagnostics, and simple quick fixes — leaving Debian maintainers without the rich IDE experience available in other ecosystems. This project significantly expands its capabilities across five areas identified from direct codebase exploration: clickable links and URL navigation in packaging files, richer hover documentation sourced from Debian Policy, improved completion quality with context-aware and deduplicated suggestions, consistent editor integration (with a focus on Helix), and formatting and structural editing support including folding ranges and auto-formatting. Each area will be delivered as an independent, tested pull request. The work integrates with debian-analyzer and the deb822-lossless crate to support multiple Debian control-file formats (debian/control, debian/copyright, debian/changelog, debian/watch ...) and lays the groundwork for real-time validation and go-to-definition across multi-file packaging trees.