refactor(backend): phase 4 - add test hooks and extend service layer

- Extract internal functions in commands/mcp.rs and commands/provider.rs
  to enable unit testing without Tauri context
- Add test hooks: set_mcp_enabled_test_hook, import_mcp_from_claude_test_hook,
  import_mcp_from_codex_test_hook, import_default_config_test_hook
- Migrate error types from String to AppError for precise error matching in tests
- Extend ProviderService with delete() method to unify Codex/Claude cleanup logic
- Add comprehensive test coverage:
  - tests/mcp_commands.rs: command-level tests for MCP operations
  - tests/provider_service.rs: service-level tests for switch/delete operations
- Run cargo fmt to fix formatting issues (EOF newlines)
- Update BACKEND_REFACTOR_PLAN.md to mark phase 3 complete

src-tauri/src/commands/settings.rs

@@ -11,8 +11,7 @@ pub async fn get_settings() -> Result<crate::settings::AppSettings, String> {
 /// 保存设置
 #[tauri::command]
 pub async fn save_settings(settings: crate::settings::AppSettings) -> Result<bool, String> {
-    crate::settings::update_settings(settings)
-        .map_err(|e| e.to_string())?;
+    crate::settings::update_settings(settings).map_err(|e| e.to_string())?;
     Ok(true)
 }