conftest: file-based credentials for auth-flow tests

Replace individual GN_TEST_EMAIL/GN_TEST_PASSWORD/GN_OAUTH2_CLIENT_ID/
GN_OAUTH2_CLIENT_SECRET env vars with GN_TEST_USERS_FILE and
GN_TEST_CLIENT_FILE, which point to the JSON files produced by the CI
test-session setup commands (create-test-users, create-test-oauth2-client).

The oauth2_credentials fixture retains the individual env var fallback so
manual runs against an existing account still work.  basic_oauth2_credentials
requires the file-based approach (basic users are only created by the CI
setup, not for manual runs).

Adds _read_json_file and _user_by_role helpers, and appends
basic_oauth2_credentials and basic_access_token fixtures.

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