FIX: Recover from guardian check when deleting reviewable users. (#17949)

Handles edge-case when a user is an admin and has an associated reviewable. Hitting this exception should be rare since we clear the reviewable when
granting staff to the user.

app/models/reviewable_user.rb

@@ -69,8 +69,8 @@ class ReviewableUser < Reviewable
         end
 
         destroyer.destroy(target, delete_args)
-      rescue UserDestroyer::PostsExistError
-        # If a user has posts, we won't delete them to preserve their content.
+      rescue UserDestroyer::PostsExistError, Discourse::InvalidAccess
+        # If a user has posts or user is an admin, we won't delete them to preserve their content.
         # However the reviewable record will be "rejected" and they will remain
         # unapproved in the database. A staff member can still approve them
         # via the admin.

spec/models/reviewable_user_spec.rb

@@ -92,7 +92,9 @@ RSpec.describe ReviewableUser, type: :model do
         expect(reviewable.target.approved_at).to be_present
         expect(reviewable.version > 0).to eq(true)
       end
+    end
 
+    context "when rejecting" do
       it "allows us to reject a user" do
         result = reviewable.perform(moderator, :delete_user, reject_reason: "reject reason")
         expect(result.success?).to eq(true)
@@ -162,6 +164,18 @@ RSpec.describe ReviewableUser, type: :model do
         expect(reviewable.rejected?).to eq(true)
         expect(reviewable.target).to be_blank
       end
+
+      it "silently transitions the reviewable if the user is an admin" do
+        reviewable.target.update!(admin: true)
+
+        result = reviewable.perform(moderator, :delete_user)
+        expect(reviewable.pending?).to eq(false)
+        expect(reviewable.rejected?).to eq(true)
+
+        reviewable.reload
+        expect(reviewable.target).to be_present
+        expect(reviewable.target.approved).to eq(false)
+      end
     end
   end