When logging in using ChatGPT using the `codex login` command, a
successful login should write a new `auth.json` file with the ChatGPT
token information. The old code attempted to retain the API key and
merge the token information into the existing `auth.json` file. With the
new simplified login mechanism, `auth.json` should have auth information
for only ChatGPT or API Key, not both.
The `codex login --api-key <key>` code path was already doing the right
thing here, but the `codex login` command was incorrect. This PR fixes
the problem and adds test cases for both commands.
900bb01486