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/lib/tasks/chat.rake

@@ -1,23 +0,0 @@
-# frozen_string_literal: true
-
-if Discourse.allow_dev_populate?
-  chat_task = Rake::Task["dev:populate"]
-  chat_task.enhance do
-    SiteSetting.chat_enabled = true
-    DiscourseDev::PublicChannel.populate!
-    DiscourseDev::DirectChannel.populate!
-    DiscourseDev::Message.populate!
-  end
-
-  desc "Generates sample content for chat"
-  task "chat:populate" => ["db:load_config"] do |_, args|
-    DiscourseDev::PublicChannel.new.populate!(ignore_current_count: true)
-    DiscourseDev::DirectChannel.new.populate!(ignore_current_count: true)
-    DiscourseDev::Message.new.populate!(ignore_current_count: true)
-  end
-
-  desc "Generates sample messages in channels"
-  task "chat:message:populate" => ["db:load_config"] do |_, args|
-    DiscourseDev::Message.new.populate!(ignore_current_count: true)
-  end
-end

plugins/chat/lib/tasks/chat_message.rake

@@ -15,7 +15,7 @@ end
 
 def rebake_uncooked_chat_messages
   puts "Rebaking uncooked chat messages on #{RailsMultisite::ConnectionManagement.current_db}"
-  uncooked = ChatMessage.uncooked
+  uncooked = Chat::Message.uncooked
 
   rebaked = 0
   total = uncooked.count
@@ -100,7 +100,7 @@ task "chat:make_channel_to_test_archiving", [:user_for_membership] => :environme
       raw: "This is some cool first post for archive stuff",
     )
     chat_channel =
-      ChatChannel.create(
+      Chat::Channel.create(
         chatable: topic,
         chatable_type: "Topic",
         name: "testing channel for archiving #{SecureRandom.hex(4)}",
@@ -112,12 +112,13 @@ task "chat:make_channel_to_test_archiving", [:user_for_membership] => :environme
 
   users = [make_test_user, make_test_user, make_test_user]
 
-  ChatChannel.transaction do
+  Chat::Channel.transaction do
     start_time = Time.now
 
     puts "creating 1039 messages for the channel"
     1039.times do
-      cm = ChatMessage.new(message: messages.sample, user: users.sample, chat_channel: chat_channel)
+      cm =
+        Chat::Message.new(message: messages.sample, user: users.sample, chat_channel: chat_channel)
       cm.cook
       cm.save!
     end
@@ -125,7 +126,7 @@ task "chat:make_channel_to_test_archiving", [:user_for_membership] => :environme
     puts "message creation done"
     puts "took #{Time.now - start_time} seconds"
 
-    UserChatChannelMembership.create(
+    Chat::UserChatChannelMembership.create(
       chat_channel: chat_channel,
       last_read_message_id: 0,
       user: User.find_by(username: user_for_membership),