fix: resolve all cargo clippy warnings

- Fix toml version requirement metadata warning
- Replace clone() with std::slice::from_ref()
- Collapse nested if statements
- Add #[allow(dead_code)] to test utility functions
- Fix unused imports and variables in tests
- Use unwrap_or_else instead of expect with format!

wtismycode-core/tests/full_pipeline.rs

@@ -139,7 +139,7 @@ fn test_cycle_detection_no_cycles() {
 
 #[test]
 fn test_renderer_produces_output() {
-    let config = Config::default();
+    let _config = Config::default();
     let model = ProjectModel::new();
     let renderer = Renderer::new();
     let result = renderer.render_architecture_md(&model, None);