DEV: properly namespace chat (#20690)

This commit main goal was to comply with Zeitwerk and properly rely on autoloading. To achieve this, most resources have been namespaced under the `Chat` module.

- Given all models are now namespaced with `Chat::` and would change the stored types in DB when using polymorphism or STI (single table inheritance), this commit uses various Rails methods to ensure proper class is loaded and the stored name in DB is unchanged, eg: `Chat::Message` model will be stored as `"ChatMessage"`, and `"ChatMessage"` will correctly load `Chat::Message` model.
- Jobs are now using constants only, eg: `Jobs::Chat::Foo` and should only be enqueued this way

Notes:
- This commit also used this opportunity to limit the number of registered css files in plugin.rb
- `discourse_dev` support has been removed within this commit and will be reintroduced later

<!-- NOTE: All pull requests should have tests (rspec in Ruby, qunit in JavaScript). If your code does not include test coverage, please include an explanation of why it was omitted. -->

plugins/chat/spec/serializer/chat/channel_serializer_spec.rb

@@ -2,7 +2,7 @@
 
 require "rails_helper"
 
-describe ChatChannelSerializer do
+describe Chat::ChannelSerializer do
   fab!(:user) { Fabricate(:user) }
   fab!(:admin) { Fabricate(:admin) }
   fab!(:chat_channel) { Fabricate(:chat_channel) }
@@ -29,10 +29,10 @@ describe ChatChannelSerializer do
       it "includes the archive status if the channel is archived and the archive record exists" do
         expect(subject.as_json.key?(:archive_completed)).to eq(false)
 
-        chat_channel.update!(status: ChatChannel.statuses[:archived])
+        chat_channel.update!(status: Chat::Channel.statuses[:archived])
         expect(subject.as_json.key?(:archive_completed)).to eq(false)
 
-        ChatChannelArchive.create!(
+        Chat::ChannelArchive.create!(
           chat_channel: chat_channel,
           archived_by: admin,
           destination_topic_title: "This will be the archive topic",

plugins/chat/spec/serializer/chat/chat_message_serializer_spec.rb

@@ -2,7 +2,7 @@
 
 require "rails_helper"
 
-describe ChatMessageSerializer do
+describe Chat::MessageSerializer do
   fab!(:chat_channel) { Fabricate(:category_channel) }
   fab!(:message_poster) { Fabricate(:user) }
   fab!(:message_1) { Fabricate(:chat_message, user: message_poster, chat_channel: chat_channel) }
@@ -79,7 +79,7 @@ describe ChatMessageSerializer do
       let(:options) { { scope: guardian, root: nil, chat_channel: message_1.chat_channel } }
 
       it "returns an empty list if the user already flagged the message" do
-        reviewable = Fabricate(:reviewable_chat_message, target: message_1)
+        reviewable = Fabricate(:chat_reviewable_message, target: message_1)
 
         serialized =
           described_class.new(
@@ -98,7 +98,7 @@ describe ChatMessageSerializer do
       end
 
       it "return available flags if staff already reviewed the previous flag" do
-        reviewable = Fabricate(:reviewable_chat_message, target: message_1)
+        reviewable = Fabricate(:chat_reviewable_message, target: message_1)
 
         serialized =
           described_class.new(

plugins/chat/spec/serializer/chat/chat_message_user_serializer_spec.rb

@@ -2,7 +2,7 @@
 
 require "rails_helper"
 
-RSpec.describe ChatMessageUserSerializer do
+RSpec.describe Chat::MessageUserSerializer do
   subject do
     user = Fabricate(:user, **params)
     guardian = Guardian.new(user)

plugins/chat/spec/serializer/chat/direct_message_serializer_spec.rb

@@ -2,7 +2,7 @@
 
 require "rails_helper"
 
-describe DirectMessageSerializer do
+describe Chat::DirectMessageSerializer do
   describe "#user" do
     it "returns you when there are two of us" do
       me = Fabricate.build(:user)

plugins/chat/spec/serializer/chat/in_reply_to_serializer_spec.rb

@@ -2,7 +2,7 @@
 
 require "rails_helper"
 
-RSpec.describe ChatInReplyToSerializer do
+RSpec.describe Chat::InReplyToSerializer do
   subject(:serializer) { described_class.new(message, scope: guardian, root: nil) }
 
   fab!(:chat_channel) { Fabricate(:chat_channel) }

plugins/chat/spec/serializer/chat/structured_channel_serializer_spec.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-RSpec.describe StructuredChannelSerializer do
+RSpec.describe Chat::StructuredChannelSerializer do
   fab!(:user1) { Fabricate(:user) }
   fab!(:guardian) { Guardian.new(user1) }
   fab!(:user2) { Fabricate(:user) }
@@ -33,7 +33,7 @@ RSpec.describe StructuredChannelSerializer do
   end
 
   def fetch_data
-    Chat::ChatChannelFetcher.structured(guardian)
+    Chat::ChannelFetcher.structured(guardian)
   end
 
   it "serializes a public channel correctly with membership embedded" do