Prevent changing work history for received reference

If the reference has already been received it shouldn't be possible to
change the name of the contact as it would then be incorrect, suggesting
a different person responded to the reference.

app/services/update_work_history_contact.rb

@@ -12,6 +12,10 @@ def initialize(work_history:, user:, name: nil, job: nil, email: nil)
   end
 
   def call
+    if reference_request.present? && reference_request.received?
+      raise InvalidState, "Reference has already been received."
+    end
+
     change_value("contact_name", name) if name.present?
 
     change_value("contact_job", job) if job.present?
@@ -27,33 +31,33 @@ def call
           contact_email_domain: email_address.host_name,
         },
       )
-    end
 
-    if email.present? && (reference_request = work_history.reference_request)
-      reference_request.regenerate_slug
+      if reference_request.present?
+        reference_request.regenerate_slug
 
-      RequestRequestable.call(
-        requestable: reference_request,
-        user:,
-        allow_already_requested: true,
-      )
+        RequestRequestable.call(
+          requestable: reference_request,
+          user:,
+          allow_already_requested: true,
+        )
 
-      ApplicationFormStatusUpdater.call(application_form:, user:)
+        ApplicationFormStatusUpdater.call(application_form:, user:)
 
-      DeliverEmail.call(
-        application_form:,
-        mailer: TeacherMailer,
-        action: :references_requested,
-        reference_requests: [reference_request],
-      )
+        DeliverEmail.call(
+          application_form:,
+          mailer: TeacherMailer,
+          action: :references_requested,
+          reference_requests: [reference_request],
+        )
+      end
     end
   end
 
   private
 
   attr_reader :work_history, :user, :name, :job, :email
 
-  delegate :application_form, to: :work_history
+  delegate :application_form, :reference_request, to: :work_history
 
   def change_value(column_name, new_value, additional_updates: {})
     old_value = work_history.send(column_name)
@@ -75,4 +79,7 @@ def create_timeline_event(column_name, old_value, new_value)
       new_value:,
     )
   end
+
+  class InvalidState < StandardError
+  end
 end