Only load authnz routes when oidc enabled

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

lib/galaxy/webapps/galaxy/buildapp.py

@@ -89,17 +89,18 @@ def app_pair(global_conf, load_app_kwds=None, wsgi_preflight=True, **kwargs):
     webapp.add_route("/activate", controller="user", action="activate")
 
     # Authentication endpoints.
-    webapp.add_route("/authnz/", controller="authnz", action="index", provider=None)
-    webapp.add_route("/authnz/{provider}/login", controller="authnz", action="login", provider=None)
-    webapp.add_route("/authnz/{provider}/callback", controller="authnz", action="callback", provider=None)
-    webapp.add_route(
-        "/authnz/{provider}/disconnect/{email}", controller="authnz", action="disconnect", provider=None, email=None
-    )
-    webapp.add_route("/authnz/{provider}/logout", controller="authnz", action="logout", provider=None)
-    webapp.add_route("/authnz/{provider}/create_user", controller="authnz", action="create_user")
-    # Returns the provider specific logout url for currently logged in provider
-    webapp.add_route("/authnz/logout", controller="authnz", action="get_logout_url")
-    webapp.add_route("/authnz/get_cilogon_idps", controller="authnz", action="get_cilogon_idps")
+    if app.config.enable_oidc:
+        webapp.add_route("/authnz/", controller="authnz", action="index", provider=None)
+        webapp.add_route("/authnz/{provider}/login", controller="authnz", action="login", provider=None)
+        webapp.add_route("/authnz/{provider}/callback", controller="authnz", action="callback", provider=None)
+        webapp.add_route(
+            "/authnz/{provider}/disconnect/{email}", controller="authnz", action="disconnect", provider=None, email=None
+        )
+        webapp.add_route("/authnz/{provider}/logout", controller="authnz", action="logout", provider=None)
+        webapp.add_route("/authnz/{provider}/create_user", controller="authnz", action="create_user")
+        # Returns the provider specific logout url for currently logged in provider
+        webapp.add_route("/authnz/logout", controller="authnz", action="get_logout_url")
+        webapp.add_route("/authnz/get_cilogon_idps", controller="authnz", action="get_cilogon_idps")
 
     # These two routes handle our simple needs at the moment
     webapp.add_route(