Write tests for token handler

pkg/lib/oauth/handler/handler_token.go

This file is complicated but we don't have unit tests for it. We need to write some tests so that we can modify the file in future easier.

- [ ] Write unit tests for each grant_type
- [ ] Some grant_type have different cases, such as authorization_code flow could issue different tokens in different cases. Should test all cases.
- [ ] e2e tests are good to have

Start with pkg/lib/oauth/handler/handler_token.go and map each grant_type branch and its cases before designing the unit tests. Done means unit tests cover every grant_type and the different authorization_code outcomes described in the issue; end-to-end tests are optional.

Written by the indexing model from the issue text.