Allow files with .md.erb for the Static Site Editor

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/223171

We want to extend Static Site Editor functionality to support more
file extensions.

changelogs/unreleased/223171_allow_erb_extension_for_sse.yml

+---
+title: Allow files with .md.erb extension for the Static Site Editor
+merge_request: 35136
+author:
+type: added

lib/gitlab/static_site_editor/config.rb

@@ -3,7 +3,7 @@
 module Gitlab
   module StaticSiteEditor
     class Config
-      SUPPORTED_EXTENSIONS = %w[.md].freeze
+      SUPPORTED_EXTENSIONS = %w[.md .md.erb].freeze
 
       def initialize(repository, ref, file_path, return_url)
         @repository = repository
@@ -42,11 +42,11 @@ module Gitlab
       end
 
       def extension_supported?
-        File.extname(file_path).in?(SUPPORTED_EXTENSIONS)
+        SUPPORTED_EXTENSIONS.any? { |ext| file_path.end_with?(ext) }
       end
 
       def file_exists?
-        commit_id.present? && repository.blob_at(commit_id, file_path).present?
+        commit_id.present? && !repository.blob_at(commit_id, file_path).nil?
       end
 
       def full_path

spec/lib/gitlab/static_site_editor/config_spec.rb

@@ -30,6 +30,22 @@ describe Gitlab::StaticSiteEditor::Config do
       )
     end
 
+    context 'when file has .md.erb extension' do
+      let(:file_path) { 'README.md.erb' }
+
+      before do
+        repository.create_file(
+          project.creator,
+          file_path,
+          '',
+          message: 'message',
+          branch_name: 'master'
+        )
+      end
+
+      it { is_expected.to include(is_supported_content: 'true') }
+    end
+
     context 'when file path is nested' do
       let(:file_path) { 'lib/README.md' }