protobuf stubs don't discriminate between unary and streaming requests

These different methods:
```protobuf
service Mu {
rpc Search (SearchRequest) returns (SearchResponse);
rpc StreamingSearch (stream SearchRequest) returns (stream SearchResponse);
}
```

generate into the same code in the stub:
```dart
$async.Future search($pb.ClientContext ctx, SearchRequest request) {
var emptyResponse = SearchResponse();
return _client.invoke(ctx, 'SearchService', 'Search', request, emptyResponse);
}
$async.Future streamingSearch($pb.ClientContext ctx, SearchRequest request) {
var emptyResponse = SearchResponse();
return _client.invoke(ctx, 'SearchService', 'StreamingSearch', request, emptyResponse);
}
```

This is incorrect, because unary and streaming requests aren't interchangeable, and the streaming request in particular can't yield multiple values or receive multiple values

No file, test, or entry point is identified in the issue. Locate the Dart protobuf stub-generation entry point and inspect how unary versus streaming RPCs are represented; done means the generated APIs distinguish both request types and support streaming values correctly.

Written by the indexing model from the issue text.