DEV: Improve test coverage of `themes:update` rake task (#26764)

This commit adds a test to ensure that settings for a theme are retained
even if saving a theme setting after a theme setting migration fails.
Prior to the fix introduced in 35bc27a36d,
failing to save the new settings will actually cause some or all of the
theme's setting to be lost.

spec/tasks/themes_spec.rb

@@ -18,7 +18,20 @@ RSpec.describe "tasks/themes" do
       }
       JSON
 
-      setup_git_repo("about.json" => about_json, "common/header.html" => "I AM A HEADER")
+      settings_yaml = <<~YAML
+      some_setting:
+        type: string
+        default: some default value
+      string_setting:
+        type: string
+        default: default value
+      YAML
+
+      setup_git_repo(
+        "about.json" => about_json,
+        "common/header.html" => "I AM A HEADER",
+        "settings.yml" => settings_yaml,
+      )
     end
 
     let(:initial_repo_url) do
@@ -31,6 +44,57 @@ RSpec.describe "tasks/themes" do
 
     after { `rm -fr #{initial_repo}` }
 
+    it "should retain a theme's settings and not update the theme when a theme's setting fails to save" do
+      theme.update_setting(:some_setting, "some setting value")
+
+      migration_content = <<~JS
+      export default function migrate(settings) {
+        const oldSetting = settings.get("string_setting");
+        settings.set("string_setting", [{}]);
+        return settings;
+      }
+      JS
+
+      settings_yaml = <<~YAML
+      string_setting:
+        type: objects
+        default: []
+        schema:
+          name: some object
+          properties:
+            title:
+              type: string
+              required: true
+      YAML
+
+      add_to_git_repo(
+        initial_repo,
+        "migrations/settings/0001-a-migration.js" => migration_content,
+        "settings.yml" => settings_yaml,
+        "common/header.html" => "I AM UPDATED HEADER",
+      )
+
+      original_remote_version = theme.remote_theme.remote_version
+      original_local_version = theme.remote_theme.local_version
+
+      stderr = capture_stderr { capture_stdout { Rake::Task["themes:update"].invoke } }
+
+      expect(stderr.chomp).to eq(
+        "[default] Failed to update 'awesome theme' (#{theme.id}): The property at JSON Pointer '/0/title' must be present.",
+      )
+
+      theme.reload
+
+      expect(theme.theme_fields.count).to eq(2)
+      expect(theme.theme_fields.where(name: "header").first.value).to eq("I AM A HEADER")
+      expect(theme.theme_settings_migrations.count).to eq(0)
+      expect(theme.remote_theme.commits_behind).to eq(0)
+      expect(theme.remote_theme.remote_version).to eq(original_remote_version)
+      expect(theme.remote_theme.local_version).to eq(original_local_version)
+      expect(theme.settings[:some_setting].value).to eq("some setting value")
+      expect(theme.settings[:string_setting].value).to eq("default value")
+    end
+
     it "should not update the theme if a theme setting migration fails during the update" do
       migration_content = <<~JS
       export default function migrate(settings) {
@@ -51,7 +115,7 @@ RSpec.describe "tasks/themes" do
 
       theme.reload
 
-      expect(theme.theme_fields.count).to eq(1)
+      expect(theme.theme_fields.count).to eq(2)
       expect(theme.theme_fields.where(name: "header").first.value).to eq("I AM A HEADER")
       expect(theme.theme_settings_migrations.count).to eq(0)
       expect(theme.remote_theme.commits_behind).to eq(0)