manual cherry-pick: [MM-67202] Validate auth method in account switch (#34981) (#35143)

* fix account authorization type switch

* improve test clarity

* refactor tests for clarity

server/i18n/en.json

@@ -4266,6 +4266,10 @@
     "id": "api.user.oauth_to_email.not_available.app_error",
     "translation": "Authentication Transfer not configured or available on this server."
   },
+  {
+    "id": "api.user.oauth_to_email.not_oauth_user.app_error",
+    "translation": "Unable to switch to email authentication because user is not using OAuth or SAML authentication."
+  },
   {
     "id": "api.user.patch_user.login_provider_attribute_set.app_error",
     "translation": "Field '{{.Field}}' must be set through user's login provider."