refactor(api): simplify app type parameter handling to single required parameter

Replace the previous dual-parameter approach (app_type/app/appType) with a single required `app: String` parameter across all Tauri commands. This change:

- Introduces unified `parse_app()` helper replacing complex `resolve_app_type()` logic
- Updates all backend commands in config, mcp, and provider modules
- Aligns frontend API calls to use consistent `app` parameter naming
- Simplifies MSW test handlers by removing optional parameter handling

This improves API clarity and reduces parameter ambiguity while maintaining backward compatibility through error handling.

src-tauri/src/lib.rs

@@ -267,12 +267,7 @@ fn switch_provider_internal(
         let app_type_str = app_type.as_str().to_string();
         let provider_id_clone = provider_id.clone();
 
-        crate::commands::switch_provider(
-            app_state.clone(),
-            Some(app_type),
-            Some(app_type_str.clone()),
-            provider_id,
-        )
+        crate::commands::switch_provider(app_state.clone(), app_type_str.clone(), provider_id)
         .map_err(AppError::Message)?;
 
         // 切换成功后重新创建托盘菜单