FIX: Validate MF strings when adding overrides

Currently, when adding translation overrides, values aren’t validated
for MF strings. This results in being able to add invalid plural keys or
even strings containing invalid syntax.

This patch addresses this issue by compiling the string when saving an
override if the key is detected as an MF one.

If there’s an error from the compiler, it’s added to the model errors,
which in turn is displayed to the user in the admin UI, helping them to
understand what went wrong.

spec/models/translation_override_spec.rb

@@ -148,6 +148,32 @@ RSpec.describe TranslationOverride do
         end
       end
     end
+
+    describe "MessageFormat translations" do
+      subject(:override) do
+        described_class.new(
+          translation_key: "admin_js.admin.user.delete_all_posts_confirm_MF",
+          locale: "en",
+        )
+      end
+
+      it do
+        is_expected.to allow_value(
+          "This has {COUNT, plural, one{one member} other{# members}}.",
+        ).for(:value).against(:base)
+      end
+      it do
+        is_expected.not_to allow_value(
+          "This has {COUNT, plural, one{one member} many{# members} other{# members}}.",
+        ).for(:value).with_message(/plural case many is not valid/, against: :base)
+      end
+      it do
+        is_expected.not_to allow_value("This has {COUNT, ").for(:value).with_message(
+          /invalid syntax/,
+          against: :base,
+        )
+      end
+    end
   end
 
   it "upserts values" do
@@ -340,4 +366,20 @@ RSpec.describe TranslationOverride do
       expect(translation.invalid_interpolation_keys).to contain_exactly("foo")
     end
   end
+
+  describe "#message_format?" do
+    subject(:override) { described_class.new(translation_key: key) }
+
+    context "when override is for a MessageFormat translation" do
+      let(:key) { "admin_js.admin.user.delete_all_posts_confirm_MF" }
+
+      it { is_expected.to be_a_message_format }
+    end
+
+    context "when override is not for a MessageFormat translation" do
+      let(:key) { "admin_js.type_to_filter" }
+
+      it { is_expected.not_to be_a_message_format }
+    end
+  end
 end