Include 404 tests for External Authenticator endpoints

[john-odonnell]

Is your feature request related to a problem? Please describe.

404 is a viable response for /authn-{service}/{service_id}/{account}/{login}/authenticate endpoints. Tests reproducing this response should be added to the existing tests.

Describe the solution you would like

One test case function for each tested authenticator:

• [ ] test_get_access_token_via_ldap_404
• [ ] test_get_access_token_via_oidc_404
• [ ] test_get_access_token_via_gcp_404

Describe alternatives you have considered

n/a

Additional context

n/a

[john-odonnell]

The 404 responses listed here are not valid responses on these endpoints. They are a remnant of a previous design decision to generalize external authenticator endpoints.