DEV: allows a context when creating a message (#25647)

The service `Chat::CreateMessage` will now accept `context_post_ids` and `context_topic_id` as params. These values represent the topic which might be visible when sending a message (for now, this is only possible when using the drawer).

The `DiscourseEvent` `chat_message_created` will now have the following signature:

```ruby
on(:chat_message_created) do | message, channel, user, meta|
  p meta[:context][:post_ids]
end
```

plugins/chat/spec/system/drawer_spec.rb

@@ -151,4 +151,56 @@ RSpec.describe "Drawer", type: :system do
       expect(drawer_page).to have_open_channel(channel)
     end
   end
+
+  context "when sending a message from topic" do
+    fab!(:topic)
+    fab!(:posts) { Fabricate.times(5, :post, topic: topic) }
+    fab!(:channel) { Fabricate(:chat_channel) }
+    fab!(:membership) do
+      Fabricate(:user_chat_channel_membership, user: current_user, chat_channel: channel)
+    end
+
+    let(:topic_page) { PageObjects::Pages::Topic.new }
+
+    context "when on a channel" do
+      it "has context" do
+        ::Chat::CreateMessage
+          .expects(:call)
+          .with do |value|
+            value["topic_id"] === topic.id.to_s &&
+              value["post_ids"] === [posts[1].id.to_s, posts[2].id.to_s, posts[3].id.to_s]
+          end
+
+        topic_page.visit_topic(topic, post_number: 3)
+        chat_page.open_from_header
+        drawer_page.open_channel(channel)
+        channel_page.send_message
+      end
+    end
+
+    context "when on a thread" do
+      before { channel.update!(threading_enabled: true) }
+
+      fab!(:thread_1) { Fabricate(:chat_thread, channel: channel) }
+
+      let(:thread_list_page) { PageObjects::Components::Chat::ThreadList.new }
+      let(:thread_page) { PageObjects::Pages::ChatThread.new }
+
+      it "has context" do
+        ::Chat::CreateMessage
+          .expects(:call)
+          .with do |value|
+            value["topic_id"] === topic.id.to_s &&
+              value["post_ids"] === [posts[1].id.to_s, posts[2].id.to_s, posts[3].id.to_s]
+          end
+
+        topic_page.visit_topic(topic, post_number: 3)
+        chat_page.open_from_header
+        drawer_page.open_channel(channel)
+        drawer_page.open_thread_list
+        thread_list_page.open_thread(thread_1)
+        thread_page.send_message
+      end
+    end
+  end
 end