DEV: Chat service object initial implementation (#19814)

This is a combined work of Martin Brennan, Loïc Guitaut, and Joffrey Jaffeux.

---

This commit implements a base service object when working in chat. The documentation is available at https://discourse.github.io/discourse/chat/backend/Chat/Service.html

Generating documentation has been made as part of this commit with a bigger goal in mind of generally making it easier to dive into the chat project.

Working with services generally involves 3 parts:

- The service object itself, which is a series of steps where few of them are specialized (model, transaction, policy)

```ruby
class UpdateAge
  include Chat::Service::Base

  model :user, :fetch_user
  policy :can_see_user
  contract
  step :update_age

  class Contract
    attribute :age, :integer
  end

  def fetch_user(user_id:, **)
    User.find_by(id: user_id)
  end

  def can_see_user(guardian:, **)
    guardian.can_see_user(user)
  end

  def update_age(age:, **)
    user.update!(age: age)
  end
end
```

- The `with_service` controller helper, handling success and failure of the service within a service and making easy to return proper response to it from the controller

```ruby
def update
  with_service(UpdateAge) do
    on_success { render_serialized(result.user, BasicUserSerializer, root: "user") }
  end
end
```

- Rspec matchers and steps inspector, improving the dev experience while creating specs for a service

```ruby
RSpec.describe(UpdateAge) do
  subject(:result) do
    described_class.call(guardian: guardian, user_id: user.id, age: age)
  end

  fab!(:user) { Fabricate(:user) }
  fab!(:current_user) { Fabricate(:admin) }

  let(:guardian) { Guardian.new(current_user) }
  let(:age) { 1 }

   it { expect(user.reload.age).to eq(age) }
end
```

Note in case of unexpected failure in your spec, the output will give all the relevant information:

```
  1) UpdateAge when no channel_id is given is expected to fail to find a model named 'user'
     Failure/Error: it { is_expected.to fail_to_find_a_model(:user) }

       Expected model 'foo' (key: 'result.model.user') was not found in the result object.

       [1/4] [model] 'user' ❌
       [2/4] [policy] 'can_see_user'
       [3/4] [contract] 'default'
       [4/4] [step] 'update_age'

       /Users/joffreyjaffeux/Code/pr-discourse/plugins/chat/app/services/update_age.rb:32:in `fetch_user': missing keyword: :user_id (ArgumentError)
       	from /Users/joffreyjaffeux/Code/pr-discourse/plugins/chat/app/services/base.rb:202:in `instance_exec'
       	from /Users/joffreyjaffeux/Code/pr-discourse/plugins/chat/app/services/base.rb:202:in `call'
       	from /Users/joffreyjaffeux/Code/pr-discourse/plugins/chat/app/services/base.rb:219:in `call'
       	from /Users/joffreyjaffeux/Code/pr-discourse/plugins/chat/app/services/base.rb:417:in `block in run!'
       	from /Users/joffreyjaffeux/Code/pr-discourse/plugins/chat/app/services/base.rb:417:in `each'
       	from /Users/joffreyjaffeux/Code/pr-discourse/plugins/chat/app/services/base.rb:417:in `run!'
       	from /Users/joffreyjaffeux/Code/pr-discourse/plugins/chat/app/services/base.rb:411:in `run'
       	from <internal:kernel>:90:in `tap'
       	from /Users/joffreyjaffeux/Code/pr-discourse/plugins/chat/app/services/base.rb:302:in `call'
       	from /Users/joffreyjaffeux/Code/pr-discourse/plugins/chat/spec/services/update_age_spec.rb:15:in `block (3 levels) in <main>'
```

plugins/chat/plugin.rb

@@ -91,6 +91,7 @@ require_relative "app/core_ext/plugin_instance.rb"
 GlobalSetting.add_default(:allow_unsecure_chat_uploads, false)
 
 after_initialize do
+  # Namespace for classes and modules parts of chat plugin
   module ::Chat
     PLUGIN_NAME = "chat"
     HAS_CHAT_ENABLED = "has_chat_enabled"
@@ -119,6 +120,7 @@ after_initialize do
          "../app/controllers/admin/admin_incoming_chat_webhooks_controller.rb",
          __FILE__,
        )
+  load File.expand_path("../app/helpers/with_service_helper.rb", __FILE__)
   load File.expand_path("../app/controllers/chat_base_controller.rb", __FILE__)
   load File.expand_path("../app/controllers/chat_controller.rb", __FILE__)
   load File.expand_path("../app/controllers/emojis_controller.rb", __FILE__)
@@ -163,6 +165,7 @@ after_initialize do
   load File.expand_path("../app/serializers/admin_chat_index_serializer.rb", __FILE__)
   load File.expand_path("../app/serializers/user_chat_message_bookmark_serializer.rb", __FILE__)
   load File.expand_path("../app/serializers/reviewable_chat_message_serializer.rb", __FILE__)
+  load File.expand_path("../app/services/base.rb", __FILE__)
   load File.expand_path("../lib/chat_channel_fetcher.rb", __FILE__)
   load File.expand_path("../lib/chat_channel_hashtag_data_source.rb", __FILE__)
   load File.expand_path("../lib/chat_mailer.rb", __FILE__)
@@ -191,6 +194,8 @@ after_initialize do
   load File.expand_path("../lib/slack_compatibility.rb", __FILE__)
   load File.expand_path("../lib/post_notification_handler.rb", __FILE__)
   load File.expand_path("../lib/secure_uploads_compatibility.rb", __FILE__)
+  load File.expand_path("../lib/endpoint.rb", __FILE__)
+  load File.expand_path("../lib/steps_inspector.rb", __FILE__)
   load File.expand_path("../app/jobs/regular/auto_manage_channel_memberships.rb", __FILE__)
   load File.expand_path("../app/jobs/regular/auto_join_channel_batch.rb", __FILE__)
   load File.expand_path("../app/jobs/regular/process_chat_message.rb", __FILE__)
@@ -207,7 +212,11 @@ after_initialize do
   load File.expand_path("../app/jobs/scheduled/auto_join_users.rb", __FILE__)
   load File.expand_path("../app/jobs/scheduled/chat_periodical_updates.rb", __FILE__)
   load File.expand_path("../app/services/chat_publisher.rb", __FILE__)
+  load File.expand_path("../app/services/trash_channel.rb", __FILE__)
+  load File.expand_path("../app/services/update_channel.rb", __FILE__)
+  load File.expand_path("../app/services/update_channel_status.rb", __FILE__)
   load File.expand_path("../app/services/chat_message_destroyer.rb", __FILE__)
+  load File.expand_path("../app/services/update_user_last_read.rb", __FILE__)
   load File.expand_path("../app/controllers/api_controller.rb", __FILE__)
   load File.expand_path("../app/controllers/api/chat_channels_controller.rb", __FILE__)
   load File.expand_path("../app/controllers/api/chat_current_user_channels_controller.rb", __FILE__)