refactor(backend): phase 3 - unify error handling and fix backup timestamp bug

Key improvements:
- Extract switch_provider_internal() returning AppError for better testability
- Fix backup mtime inheritance: use read+write instead of fs::copy to ensure latest backup survives cleanup
- Add 15+ integration tests covering provider commands, atomic writes, and rollback scenarios
- Expose write_codex_live_atomic, AppState, and test hooks in public API
- Extract tests/support.rs with isolated HOME and mutex utilities

Test coverage:
- Provider switching with live config backfill and MCP sync
- Codex atomic write success and failure rollback
- Backup retention policy with proper mtime ordering
- Negative cases: missing auth field, invalid provider ID

src-tauri/src/import_export.rs

@@ -28,8 +28,8 @@ pub fn create_backup(config_path: &PathBuf) -> Result<String, AppError> {
 
     let backup_path = backup_dir.join(format!("{}.json", backup_id));
 
-    // 复制配置文件到备份
-    fs::copy(config_path, &backup_path).map_err(|e| AppError::io(&backup_path, e))?;
+    let contents = fs::read(config_path).map_err(|e| AppError::io(config_path, e))?;
+    fs::write(&backup_path, contents).map_err(|e| AppError::io(&backup_path, e))?;
 
     // 备份完成后清理旧的备份文件（仅保留最近 MAX_BACKUPS 份）
     cleanup_old_backups(&backup_dir, MAX_BACKUPS)?;