refactor(next-auth): modernize URL handling and remove legacy oauth dependency

Technical debt identified during v3.29.11 audit:
- Refactor all existing references to `baseUrl` and `basePath` for better consistency.
- Refactor `oauth/client.js` to remove the dependency on the legacy `oauth` package.
- Refactor monkey patching in `OAuth2.getOAuthAccessToken()` and `OAuth2.get()`.