fix: Annotaterb model annotation incomplete migration (#13132)

This pull request fixes the model annotation tooling due to previous
incomplete migration from `annotate` to `annotaterb` gem (#12600). It
also improves the handling of serialized values in the
`InstallationConfig` model by ensuring a default value is set,
simplifying the code, and removing a workaround for YAML
deserialization.

**Annotation tooling updates:**

* Added `.annotaterb.yml` to configure the `annotate_rb` gem with
project-specific options, centralizing annotation settings.
* Replaced the custom `auto_annotate_models.rake` task with the standard
rake task from `annotate_rb`, and added `lib/tasks/annotate_rb.rake` to
load annotation tasks in development environments.
[[1]](diffhunk://#diff-9450d2359e45f1db407b3871dde787a25d60bb721aed179a65ffd2692e95fb4bL1-L61)
[[2]](diffhunk://#diff-578cdfc7ad56637e42472ea891ea286dff8803d9a1750afdbfeafec164d9b8b2R1-R8)

**Model serialization improvements:**

* Updated the `InstallationConfig` model to set a default value for the
`serialized_value` attribute, ensuring it always has a hash with
indifferent access and removing the need for a deserialization
workaround in the `value` method.
[[1]](diffhunk://#diff-b4bdde42c1ad0f584073818bd43dbd865b1b3b50d4701b131979f900d7c68297L22-R22)
[[2]](diffhunk://#diff-b4bdde42c1ad0f584073818bd43dbd865b1b3b50d4701b131979f900d7c68297L36-L39)

---------

Co-authored-by: Sojan Jose <sojan@pepalo.com>

spec/models/installation_config_spec.rb

@@ -3,5 +3,17 @@
 require 'rails_helper'
 
 RSpec.describe InstallationConfig do
+  subject(:installation_config) { described_class.new(name: 'INSTALLATION_NAME') }
+
   it { is_expected.to validate_presence_of(:name) }
+
+  describe 'new record defaults' do
+    it 'initializes serialized_value with indifferent access' do
+      expect(installation_config.serialized_value).to eq({}.with_indifferent_access)
+    end
+
+    it 'returns nil for value before assignment' do
+      expect(installation_config.value).to be_nil
+    end
+  end
 end