fix(providers/fab): restore OAuth callback route exposure

[tschroeder-zendesk]

Restore OAuth callback route registration on CustomAuthOAuthView.

CustomAuthOAuthView.oauth_authorized overrides FAB's AuthOAuthView.oauth_authorized, but without re-applying the @expose("/oauth-authorized/<provider>") decorator the callback endpoint is not registered on the custom view. This breaks URL generation for CustomAuthOAuthView.oauth_authorized during OAuth login.

This PR re-adds the route decorator and includes a regression unit test asserting the callback route remains exposed.

Fixes #62028

Testing

• source .venv/bin/activate && ruff check providers/fab/src/airflow/providers/fab/auth_manager/views/auth_oauth.py providers/fab/tests/unit/fab/auth_manager/views/test_auth_oauth.py
• breeze run --python 3.12 --platform linux/amd64 --tty disabled --skip-image-upgrade-check -- python -m pytest providers/fab/tests/unit/fab/auth_manager/views/test_auth_oauth.py -q

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