feat: Allow users to mark a conversation as unread (#5924)

Allow users to mark conversations as unread.
Loom video: https://www.loom.com/share/ab70552d3c9c48b685da7dfa64be8bb3

fixes: #5552

Co-authored-by: Pranav Raj S <pranav@chatwoot.com>

spec/controllers/api/v1/accounts/conversations_controller_spec.rb

@@ -508,6 +508,38 @@ RSpec.describe 'Conversations API', type: :request do
     end
   end
 
+  describe 'POST /api/v1/accounts/{account.id}/conversations/:id/unread' do
+    let(:conversation) { create(:conversation, account: account) }
+
+    context 'when it is an unauthenticated user' do
+      it 'returns unauthorized' do
+        post "/api/v1/accounts/#{account.id}/conversations/#{conversation.display_id}/unread"
+
+        expect(response).to have_http_status(:unauthorized)
+      end
+    end
+
+    context 'when it is an authenticated user' do
+      let(:agent) { create(:user, account: account, role: :agent) }
+
+      before do
+        create(:inbox_member, user: agent, inbox: conversation.inbox)
+        create(:message, conversation: conversation, account: account, inbox: conversation.inbox, content: 'Hello', message_type: 'incoming')
+      end
+
+      it 'updates last seen' do
+        post "/api/v1/accounts/#{account.id}/conversations/#{conversation.display_id}/unread",
+             headers: agent.create_new_auth_token,
+             as: :json
+
+        expect(response).to have_http_status(:success)
+        last_seen_at = conversation.messages.incoming.last.created_at - 1.second
+        expect(conversation.reload.agent_last_seen_at).to eq(last_seen_at)
+        expect(conversation.reload.assignee_last_seen_at).to eq(last_seen_at)
+      end
+    end
+  end
+
   describe 'POST /api/v1/accounts/{account.id}/conversations/:id/mute' do
     let(:conversation) { create(:conversation, account: account) }