[24.1] Only load authnz routes when oidc enabled

[mvdbeek]

We don't construct the necessary manager, so no use exposing API routes that won't work:

        self.authnz_manager = None
        if self.config.enable_oidc:
            from galaxy.authnz import managers

            self.authnz_manager = managers.AuthnzManager(
                self, self.config.oidc_config_file, self.config.oidc_backends_config_file
            )

Fixes #18682:

AttributeError: 'NoneType' object has no attribute 'get_allowed_idps'
(2 additional frame(s) were not displayed)
...
  File "galaxy/web/framework/middleware/statsd.py", line 29, in __call__
    req = self.application(environ, start_response)
  File "galaxy/web/framework/base.py", line 176, in __call__
    return self.handle_request(request_id, path_info, environ, start_response)
  File "galaxy/web/framework/base.py", line 271, in handle_request
    body = method(trans, **kwargs)
  File "galaxy/web/framework/decorators.py", line 74, in call_and_format
    rval = func(self, trans, *args, **kwargs)
  File "galaxy/webapps/galaxy/controllers/authnz.py", line 210, in get_cilogon_idps
    if allowed_idps := trans.app.authnz_manager.get_allowed_idps():

Uncaught Exception

How to test the changes?

(Select all options that apply)

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.