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/lib/service_runner_spec.rb

@@ -1,12 +1,12 @@
 # frozen_string_literal: true
 
-RSpec.describe Chat::ServiceRunner do
+RSpec.describe ServiceRunner do
   class SuccessService
-    include Chat::Service::Base
+    include Service::Base
   end
 
   class FailureService
-    include Chat::Service::Base
+    include Service::Base
 
     step :fail_step
 
@@ -16,7 +16,7 @@ RSpec.describe Chat::ServiceRunner do
   end
 
   class FailedPolicyService
-    include Chat::Service::Base
+    include Service::Base
 
     policy :test
 
@@ -26,7 +26,7 @@ RSpec.describe Chat::ServiceRunner do
   end
 
   class SuccessPolicyService
-    include Chat::Service::Base
+    include Service::Base
 
     policy :test
 
@@ -36,7 +36,7 @@ RSpec.describe Chat::ServiceRunner do
   end
 
   class FailedContractService
-    include Chat::Service::Base
+    include Service::Base
 
     class Contract
       attribute :test
@@ -47,13 +47,13 @@ RSpec.describe Chat::ServiceRunner do
   end
 
   class SuccessContractService
-    include Chat::Service::Base
+    include Service::Base
 
     contract
   end
 
   class FailureWithModelService
-    include Chat::Service::Base
+    include Service::Base
 
     model :fake_model, :fetch_fake_model
 
@@ -65,7 +65,7 @@ RSpec.describe Chat::ServiceRunner do
   end
 
   class SuccessWithModelService
-    include Chat::Service::Base
+    include Service::Base
 
     model :fake_model, :fetch_fake_model
 
@@ -77,7 +77,7 @@ RSpec.describe Chat::ServiceRunner do
   end
 
   class FailureWithCollectionModelService
-    include Chat::Service::Base
+    include Service::Base
 
     model :fake_model, :fetch_fake_model
 
@@ -89,7 +89,7 @@ RSpec.describe Chat::ServiceRunner do
   end
 
   class SuccessWithCollectionModelService
-    include Chat::Service::Base
+    include Service::Base
 
     model :fake_model, :fetch_fake_model
 
@@ -109,7 +109,7 @@ RSpec.describe Chat::ServiceRunner do
     let(:actions) { "proc {}" }
     let(:object) do
       Class
-        .new(Chat::Api) do
+        .new(Chat::ApiController) do
           def request
             OpenStruct.new
           end
@@ -126,7 +126,7 @@ RSpec.describe Chat::ServiceRunner do
 
     it "runs the provided service in the context of a controller" do
       runner
-      expect(result).to be_a Chat::Service::Base::Context
+      expect(result).to be_a Service::Base::Context
       expect(result).to be_a_success
     end