Remove usage of constant

Unnecessary usage of constants is a documented antipattern.  See
https://bit.ly/30eeD8R

app/services/static_site_editor/config_service.rb

@@ -58,7 +58,7 @@ module StaticSiteEditor
 
     def load_generated_config
       Gitlab::StaticSiteEditor::Config::GeneratedConfig.new(
-        project.repository,
+        repository,
         ref,
         params.fetch(:path),
         params[:return_url]
@@ -75,7 +75,7 @@ module StaticSiteEditor
     end
 
     def yaml_from_repo
-      project.repository.blob_data_at(ref, static_site_editor_config_file)
+      repository.blob_data_at(ref, static_site_editor_config_file)
     rescue GRPC::NotFound
       # Return nil in the case of a GRPC::NotFound exception, so the default config will be used.
       # Allow any other unexpected exception will be tracked and re-raised.

lib/gitlab/static_site_editor/config/generated_config.rb

@@ -4,8 +4,6 @@ module Gitlab
   module StaticSiteEditor
     module Config
       class GeneratedConfig
-        SUPPORTED_EXTENSIONS = %w[.md].freeze
-
         def initialize(repository, ref, path, return_url)
           @repository = repository
           @ref = ref
@@ -35,8 +33,12 @@ module Gitlab
 
         delegate :project, to: :repository
 
+        def supported_extensions
+          %w[.md].freeze
+        end
+
         def commit_id
-          repository.commit(ref)&.id if ref
+          repository.commit(ref)&.id
         end
 
         def supported_content?
@@ -50,7 +52,7 @@ module Gitlab
         def extension_supported?
           return true if path.end_with?('.md.erb') && Feature.enabled?(:sse_erb_support, project)
 
-          SUPPORTED_EXTENSIONS.any? { |ext| path.end_with?(ext) }
+          supported_extensions.any? { |ext| path.end_with?(ext) }
         end
 
         def file_exists?

spec/lib/gitlab/static_site_editor/config/generated_config_spec.rb

@@ -132,5 +132,11 @@ RSpec.describe Gitlab::StaticSiteEditor::Config::GeneratedConfig do
 
       it { is_expected.to include(return_url: nil) }
     end
+
+    context 'when a commit for the ref cannot be found' do
+      let(:ref) { 'nonexistent-ref' }
+
+      it { is_expected.to include(commit_id: nil) }
+    end
   end
 end