feat(adapter): surface low‑level network error details; attach original error via cause (#6982)

* feat(adapter): surface low‑level network error details; attach original error via `cause`

Node http adapter:
- Promote low-level `err.code` to `AxiosError.code`, prefixing message (e.g. `ECONNREFUSED – …`)
- Keep original error on standard `Error.cause`

XHR adapter:
- Preserve browser `ProgressEvent` on `error.event`
- Use event message when available

Tests:
- Add Node ESM tests under `test/unit/adapters` to assert `code` and `cause` behavior

Types:
- Ensure `AxiosError.cause?: unknown` and `event?: ProgressEvent` are present

* fix(adapter): use fs instead of fs/promises for sync file read in tests to fix GitHub Actions

lib/adapters/xhr.js

@@ -104,15 +104,18 @@ export default isXHRAdapterSupported && function (config) {
     };
 
     // Handle low level network errors
-    request.onerror = function handleError() {
-      // Real errors are hidden from us by the browser
-      // onerror should only fire if it's a network error
-      reject(new AxiosError('Network Error', AxiosError.ERR_NETWORK, config, request));
-
-      // Clean up request
-      request = null;
+  request.onerror = function handleError(event) {
+       // Browsers deliver a ProgressEvent in XHR onerror
+       // (message may be empty; when present, surface it)
+       // See https://developer.mozilla.org/docs/Web/API/XMLHttpRequest/error_event
+       const msg = event && event.message ? event.message : 'Network Error';
+       const err = new AxiosError(msg, AxiosError.ERR_NETWORK, config, request);
+       // attach the underlying event for consumers who want details
+       err.event = event || null;
+       reject(err);
+       request = null;
     };
-
+    
     // Handle timeout
     request.ontimeout = function handleTimeout() {
       let timeoutErrorMessage = _config.timeout ? 'timeout of ' + _config.timeout + 'ms exceeded' : 'timeout exceeded';