DEV: adds first_messages/last_messages to thread SDK (#26861)

This commit introduces several enhancements to the ChatSDK module, aiming to improve the functionality and usability of chat thread interactions. Here's what has been changed and added:

1. **New Method: `first_messages`:**
   - Added a method to retrieve the first set of messages from a specified chat thread.
   - This method is particularly useful for fetching initial messages when entering a chat thread.
   - Parameters include `thread_id`, `guardian`, and an optional `page_size` which defaults to 10.
   - Usage example added to demonstrate fetching the first 15 messages from a thread.

2. **New Method: `last_messages`:**
   - Added a method to retrieve the last set of messages from a specified chat thread.
   - This method supports reverse pagination, where the user may want to see the most recent messages first.
   - Similar to `first_messages`, it accepts `thread_id`, `guardian`, and an optional `page_size` parameter, defaulting to 10.
   - Usage example provided to illustrate fetching the last 20 messages from a thread.

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

@@ -5,12 +5,7 @@ describe ChatSDK::Thread do
     fab!(:thread_1) { Fabricate(:chat_thread) }
 
     let(:params) do
-      {
-        title: "New Title",
-        channel_id: thread_1.channel_id,
-        thread_id: thread_1.id,
-        guardian: Discourse.system_user.guardian,
-      }
+      { title: "New Title", thread_id: thread_1.id, guardian: Discourse.system_user.guardian }
     end
 
     it "changes the title" do
@@ -23,7 +18,9 @@ describe ChatSDK::Thread do
       it "fails" do
         params.delete(:thread_id)
 
-        expect { described_class.update_title(**params) }.to raise_error("Thread can't be blank")
+        expect { described_class.update_title(**params) }.to raise_error(
+          "missing keyword: :thread_id",
+        )
       end
     end
 
@@ -67,6 +64,67 @@ describe ChatSDK::Thread do
         )
       end
     end
+  end
+
+  describe ".first_messages" do
+    fab!(:thread_1) { Fabricate(:chat_thread) }
+    fab!(:messages) do
+      Fabricate.times(5, :chat_message, thread: thread_1, chat_channel: thread_1.channel)
+    end
+
+    let(:params) { { thread_id: thread_1.id, guardian: Discourse.system_user.guardian } }
+
+    it "returns messages" do
+      expect(described_class.first_messages(**params)).to eq([thread_1.original_message, *messages])
+    end
+  end
+
+  describe ".last_messages" do
+    fab!(:thread_1) { Fabricate(:chat_thread) }
+    fab!(:messages) do
+      Fabricate.times(
+        5,
+        :chat_message,
+        thread: thread_1,
+        chat_channel: thread_1.channel,
+        use_service: true,
+      )
+    end
+
+    let(:params) do
+      { thread_id: thread_1.id, guardian: Discourse.system_user.guardian, page_size: 5 }
+    end
+
+    it "returns messages" do
+      expect(described_class.last_messages(**params)).to eq([*messages])
+    end
+  end
+
+  describe ".messages" do
+    fab!(:thread_1) { Fabricate(:chat_thread) }
+    fab!(:messages) do
+      Fabricate.times(
+        5,
+        :chat_message,
+        thread: thread_1,
+        chat_channel: thread_1.channel,
+        use_service: true,
+      )
+    end
+
+    let(:params) { { thread_id: thread_1.id, guardian: Discourse.system_user.guardian } }
+
+    it "returns messages" do
+      expect(described_class.messages(**params)).to eq([thread_1.original_message, *messages])
+    end
+
+    describe "page_size:" do
+      before { params[:page_size] = 2 }
+
+      it "limits returned messages" do
+        expect(described_class.messages(**params)).to eq([thread_1.original_message, messages[0]])
+      end
+    end
 
     context "when target_message doesn’t exist" do
       it "fails" do