flatc generates invalid C++ code with `--grpc-callback-api`

When the `--grpc-callback-api` option is used, `flatc` generates invalid C++ code that fails to compile.

The `grpctest` test project fails with the following error:

```
2>W:\Dev\flatbuffers\tests\monster_test.grpc.fb.h(361,23): error C2039: 'CallbackService': is not a member of 'MyGame::Example::MonsterStorage'
2>(compiling source file '../../Dev/flatbuffers/tests/monster_test.grpc.fb.cc')
2> W:\Dev\flatbuffers\tests\monster_test.grpc.fb.h(33,7):
2> see declaration of 'MyGame::Example::MonsterStorage'
```

After inspecting the generated monster_test.grpc.fb.h file, it appears that the declaration of CallbackService is emitted in the wrong scope.

The generated code effectively declares:

`MyGame::Example::MonsterStorage::StubInterface::CallbackService`

while it should be declared as:

`MyGame::Example::MonsterStorage::CallbackService`

I verified this behavior both on the current master branch and on commit deb3d934 (the commit that originally introduced this functionality). The problem is reproducible in both cases.

From what I can see, the fix appears to be trivial: the generated `class CallbackService;` line simply needs to be swapped with the following line containing the closing brace, so that `CallbackService` is declared in the scope of `MonsterStorage` rather than inside `StubInterface`.

## Expected behavior

flatc --grpc-callback-api should generate valid C++ code, and the grpctest project should compile successfully.

## Actual behavior

Generated code places CallbackService in the wrong scope, causing compilation errors.