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

index.d.ts

@@ -418,7 +418,8 @@ export class AxiosError<T = unknown, D = any> extends Error {
   isAxiosError: boolean;
   status?: number;
   toJSON: () => object;
-  cause?: Error;
+  cause?: unknown;
+  event?: BrowserProgressEvent;
   static from<T = unknown, D = any>(
     error: Error | unknown,
     code?: string,