fixup: Use UntrustedString and simplify LSPSUrl API

- Replace String fields with UntrustedString wrapper
- Change parse to take owned String and return Err(())
- Simplify error handling, drop free‑form error messages
- Add is_https(), url_length(), is_public() helpers
- Make url() public, remove scheme()/host() getters
- Clean up parsing logic to avoid extra allocations
- Update tests to use .to_string() and new API
- Add proptest for robust property‑based testing
- Remove redundant comments and dead code

Author: martinsaposnic