FIX: serializes interaction for direct messages (#29844)

Prior to this fix it would cause an error as we need to pass the user to get the title of the channel.

This commit also adds a test for message interaction serializer.

plugins/chat/spec/serializer/chat/message_interaction_serializer_spec.rb

@@ -0,0 +1,54 @@
+# frozen_string_literal: true
+RSpec.describe Chat::MessageInteractionSerializer do
+  subject(:serializer) do
+    interaction =
+      Fabricate(
+        :chat_message_interaction,
+        message:,
+        user:,
+        action: message.blocks.first["elements"].first,
+      )
+    described_class.new(interaction, scope: Guardian.new(user), root: false)
+  end
+
+  fab!(:user)
+
+  let(:message) do
+    Fabricate(
+      :chat_message,
+      chat_channel: channel,
+      user: Discourse.system_user,
+      blocks: [
+        {
+          type: "actions",
+          elements: [
+            { type: "button", text: { type: "plain_text", text: "Like" }, action_id: "like" },
+          ],
+        },
+      ],
+    )
+  end
+  let(:message_id) { message.id }
+  let(:params) { { message_id:, action_id: "like" } }
+
+  before do
+    SiteSetting.chat_allowed_groups = Group::AUTO_GROUPS[:everyone]
+    SiteSetting.direct_message_enabled_groups = Group::AUTO_GROUPS[:everyone]
+  end
+
+  context "when interaction's channel is private" do
+    let(:channel) { Fabricate(:direct_message_channel, users: [user, Fabricate(:user)]) }
+
+    it "serializes the interaction" do
+      expect(serializer.as_json).to match_response_schema("message_interaction")
+    end
+  end
+
+  context "when interaction's channel is public" do
+    let(:channel) { Fabricate(:chat_channel) }
+
+    it "serializes the interaction" do
+      expect(serializer.as_json).to match_response_schema("message_interaction")
+    end
+  end
+end