fix: keep the request future in the stream pipeline for ResponseType.stream

[Yusufihsangorgel]

Closes #907.

The problem

Generated methods for @DioResponseType(ResponseType.stream) awaited the
request future inside an async* body:

final _result = await _dio.fetch<ResponseBody>(_options);
final _value = _result.data!.stream;
yield* _value;

If the stream subscription is cancelled while execution is suspended at that
await, a later error from the pending future is no longer delivered through
the stream listener lifecycle and can surface as an unhandled zone error. In
practice that shows up as noisy false positives in global error collectors, even
when the consumer is handling stream errors correctly.

The fix

Emit the fetch without await and expand it through the stream pipeline, so the
future stays inside the stream:

final _result = _dio.fetch<ResponseBody>(_options);
final _value = _result.asStream().asyncExpand(
  (response) => response.data!.stream,
);
yield* _value;

Stream<String> gets the same shape with .map(utf8.decode) inside the expand.
This is the change proposed in the issue.

Tests

Updated the two ResponseType.stream codegen expectations, for
Stream<Uint8List> and for Stream<String>, to the new generated output.
Generator suite green (202 tests), dart analyze clean, formatted.