Don't set started at on preliminary assessment sections

This doesn't count as part of the assessment, so we shouldn't be
recording the started at date of the assessment as part of the
preliminary check.

app/services/update_assessment_section.rb

@@ -76,7 +76,7 @@ def create_timeline_event(old_state:)
   end
 
   def update_assessment_started_at
-    return if assessment.started_at
+    return if assessment.started_at || assessment_section.preliminary
     assessment.update!(started_at: Time.zone.now)
   end
 

spec/services/update_assessment_section_spec.rb

@@ -121,7 +121,15 @@
     context "with an existing assessment started at" do
       before { assessment.update!(started_at: Date.new(2021, 1, 1)) }
 
-      it "doesn't change the assessor" do
+      it "doesn't change the started" do
+        expect { subject }.to_not change(assessment, :started_at)
+      end
+    end
+
+    context "with a preliminary assessment section" do
+      before { assessment_section.update!(preliminary: true) }
+
+      it "doesn't change the started" do
         expect { subject }.to_not change(assessment, :started_at)
       end
     end