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/app_store.rs

@@ -66,7 +66,10 @@ pub fn get_app_config_dir_from_store(app: &tauri::AppHandle) -> Option<PathBuf>
             Some(path)
         }
         Some(_) => {
-            log::warn!("Store 中的 {} 类型不正确，应为字符串", STORE_KEY_APP_CONFIG_DIR);
+            log::warn!(
+                "Store 中的 {} 类型不正确，应为字符串",
+                STORE_KEY_APP_CONFIG_DIR
+            );
             None
         }
         None => None,