Indicate WP subject as non-writable when generated

Automatic subject generation should prevent users from
editing the subject directly. Instead it can only be updated
automatically, when other fields change in that case.

lib/api/v3/work_packages/schema/base_work_package_schema.rb

@@ -53,6 +53,7 @@ def available_custom_fields
 
           def writable?(property)
             property = property.to_s
+            return false if property == "subject" && type&.replacement_pattern_defined_for?(:subject)
 
             # Special case for milestones + date property
             property = "start_date" if property == "date" && milestone?

spec/lib/api/v3/work_packages/schema/specific_work_package_schema_spec.rb

@@ -299,6 +299,16 @@
         expect(subject).to be_writable(:priority)
       end
     end
+
+    describe "subject" do
+      it { is_expected.to be_writable(:subject) }
+
+      context "when the type has automatic subject generation enabled" do
+        let(:type) { build_stubbed(:type, patterns: { subject: { blueprint: "Hello world", enabled: true } }) }
+
+        it { is_expected.not_to be_writable(:subject) }
+      end
+    end
   end
 
   describe "#assignable_custom_field_values" do

spec/lib/api/v3/work_packages/schema/typed_work_package_schema_spec.rb

@@ -83,6 +83,18 @@
     it "finish date is writable" do
       expect(subject).to be_writable(:due_date)
     end
+
+    it "subject is writable" do
+      expect(subject).to be_writable(:subject)
+    end
+
+    context "when the type has automatic subject generation enabled" do
+      let(:type) { create(:type, patterns: { subject: { blueprint: "Hello world", enabled: true } }) }
+
+      it "subject is not writable" do
+        expect(subject).not_to be_writable(:subject)
+      end
+    end
   end
 
   describe "#milestone?" do