MimeMultipartTransformer: active part stream hangs indefinitely on source stream error or unexpected done

[iv-re]

When using MimeMultipartTransformer to parse a multipart MIME stream, if the source stream encounters an error or terminates unexpectedly (closes before the final boundary is read), the stream representing the active part (_multipartController) remains open, causing any listener of that part to hang indefinitely.

Here is a standalone example that hangs indefinitely (because of the keep-alive timer):

import 'dart:async';
import 'package:mime/mime.dart';

void main() async {
  // Keep the event loop alive so the process hangs when the bug is present
  final keepAlive = Timer.periodic(const Duration(seconds: 1), (timer) {});

  final controller = StreamController<List<int>>();
  final transformer = MimeMultipartTransformer('xxx');
  final stream = controller.stream.transform(transformer);

  // Start a part, but don't finish it
  controller.add('--xxx\r\n'.codeUnits);
  controller.add('Header: value\r\n\r\npart data'.codeUnits);

  scheduleMicrotask(() {
    print('Closing source stream prematurely...');
    controller.close();
  });

  await for (final part in stream) {
    await for (final bytes in part) {
      print('Bytes: $bytes');
    }
  }
  
  keepAlive.cancel();
}

The active part stream should propagate the error/exception and close itself so that active listeners terminate immediately.