Add missing await rather than removing async

Add missing await rather than removing async

Fix async_return_with_no_await by adding the missing await and keeping async, because the next change will likely need another await anyway.

When async_return_with_no_await is mentioned, prefer adding the missing await to the returned future. Keep the function's async modifier instead of removing it to silence the lint.

Keeping async makes later refactoring easier: new functionality will often need another await. Removing async now forces a second edit to the function signature, potentially many lines above, when that happens.

Prefer:

Future<Result> loadResult() async {
  return await fetchResult();
}

Do not fix the diagnostic by changing that function to:

Future<Result> loadResult() {
  return fetchResult();
}

Apply the same preference to delegated calls in branches and callbacks. Preserve existing error handling and validate behavior when adding an await affects timing or execution zones; do not blindly apply a mechanical fix that breaks a test. This rule is a preference for fixing the named diagnostic, not a requirement to make every future-returning function async.