chore(backend): satisfy cargo fmt and clippy --all-targets

- Apply rustfmt diffs in claude_desktop_config.rs
- Allow needless_return on current_platform_paths (cfg-mirrored arms)
- Allow too_many_arguments on RequestForwarder::forward
- Replace `let mut + reassign` with struct literals in tests
  (settings, backup, provider, response_processor)
- Use Path::new instead of PathBuf::from to fix cmp_owned in misc tests
- Replace 3.14 with 3.5 in config test to avoid approx_constant lint

src-tauri/src/proxy/response_processor.rs

@@ -894,9 +894,11 @@ mod tests {
         db.set_pricing_model_source(app_type, "response").await?;
         seed_pricing(&db)?;
 
-        let mut meta = ProviderMeta::default();
-        meta.cost_multiplier = Some("2".to_string());
-        meta.pricing_model_source = Some("request".to_string());
+        let meta = ProviderMeta {
+            cost_multiplier: Some("2".to_string()),
+            pricing_model_source: Some("request".to_string()),
+            ..ProviderMeta::default()
+        };
         insert_provider(&db, "provider-1", app_type, meta)?;
 
         let state = build_state(db.clone());