DEV: start/stop reply implementation (#29542)

* DEV: join/leave presence chat-reply when streaming

This commit ensures that starting/stopping a chat message with the streaming option will automatically make the creator of the message as present in the chat-reply channel.

* implements start/stop reply

* not needed

plugins/chat/spec/lib/chat_sdk/channel_spec.rb

@@ -38,4 +38,90 @@ describe ChatSDK::Channel do
       end
     end
   end
+
+  describe ".start_reply" do
+    fab!(:channel_1) { Fabricate(:chat_channel, threading_enabled: true) }
+    fab!(:thread_1) { Fabricate(:chat_thread, channel: channel_1) }
+
+    let(:params) do
+      { channel_id: channel_1.id, thread_id: thread_1.id, guardian: Discourse.system_user.guardian }
+    end
+
+    it "starts a reply" do
+      client_id = nil
+      expect { client_id = described_class.start_reply(**params) }.to change {
+        PresenceChannel.new("/chat-reply/#{channel_1.id}/thread/#{thread_1.id}").count
+      }.by(1)
+
+      expect(client_id).to be_present
+    end
+
+    context "when the channel doesn't exist" do
+      it "fails" do
+        params[:channel_id] = -999
+
+        expect { described_class.start_reply(**params) }.to raise_error(
+          "Chat::Channel or Chat::Thread not found.",
+        )
+      end
+    end
+
+    context "when the thread doesn't exist" do
+      it "fails" do
+        params[:thread_id] = -999
+
+        expect { described_class.start_reply(**params) }.to raise_error(
+          "Chat::Channel or Chat::Thread not found.",
+        )
+      end
+    end
+  end
+
+  describe ".stop_reply" do
+    fab!(:user) { Fabricate(:user) }
+    fab!(:channel_1) { Fabricate(:chat_channel, threading_enabled: true) }
+    fab!(:thread_1) { Fabricate(:chat_thread, channel: channel_1) }
+    fab!(:client_id) do
+      described_class.start_reply(
+        channel_id: channel_1.id,
+        thread_id: thread_1.id,
+        guardian: user.guardian,
+      )
+    end
+
+    let(:params) do
+      {
+        channel_id: channel_1.id,
+        thread_id: thread_1.id,
+        client_id: client_id,
+        guardian: user.guardian,
+      }
+    end
+
+    it "stops a reply" do
+      expect { described_class.stop_reply(**params) }.to change {
+        PresenceChannel.new("/chat-reply/#{channel_1.id}/thread/#{thread_1.id}").count
+      }.by(-1)
+    end
+
+    context "when the channel doesn't exist" do
+      it "fails" do
+        params[:channel_id] = -999
+
+        expect { described_class.stop_reply(**params) }.to raise_error(
+          "Chat::Channel or Chat::Thread not found.",
+        )
+      end
+    end
+
+    context "when the thread doesn't exist" do
+      it "fails" do
+        params[:thread_id] = -999
+
+        expect { described_class.stop_reply(**params) }.to raise_error(
+          "Chat::Channel or Chat::Thread not found.",
+        )
+      end
+    end
+  end
 end