Fix issue reopen Mattermost / Slack message

When an issue is reopened, the action is 'reopen', but the state is 'opened' (as
we don't have a separate 'reopened' state any more).

Because we checked the action in one method and the state in another, this lead
to a weird case where the mesage neither linked to the issue, nor contained an
attachment with its details. Just checking the action is fine, as it's the most
granular.

app/models/project_services/chat_message/issue_message.rb

@@ -39,7 +39,7 @@ module ChatMessage
     private
 
     def message
-      if state == 'opened'
+      if opened_issue?
         "[#{project_link}] Issue #{state} by #{user_combined_name}"
       else
         "[#{project_link}] Issue #{issue_link} #{state} by #{user_combined_name}"

changelogs/unreleased/39791-when-reopening-an-issue-the-mattermost-notification-has-no-context-to-the-issue.yml

+---
+title: Include link to issue in reopen message for Slack and Mattermost notifications
+merge_request:
+author:
+type: fixed

spec/models/project_services/chat_message/issue_message_spec.rb

@@ -66,6 +66,19 @@ describe ChatMessage::IssueMessage do
         expect(subject.attachments).to be_empty
       end
     end
+
+    context 'reopen' do
+      before do
+        args[:object_attributes][:action] = 'reopen'
+        args[:object_attributes][:state] = 'opened'
+      end
+
+      it 'returns a message regarding reopening of issues' do
+        expect(subject.pretext)
+          .to eq('[<http://somewhere.com|project_name>] Issue <http://url.com|#100 Issue title> opened by Test User (test.user)')
+        expect(subject.attachments).to be_empty
+      end
+    end
   end
 
   context 'with markdown' do