DEV: Introduce run_theme_migration spec helper in test environment (#26845)

This commit introduces the `run_theme_migration` spec helper to allow
theme developers to write RSpec tests for theme migrations. For example,
this allows the following RSpec test to be written in themes:

```
RSpec.describe "0003-migrate-small-links-setting migration" do
  let!(:theme) { upload_theme_component }

  it "should set target property to `_blank` if previous target component is not valid or empty" do
    theme.theme_settings.create!(
      name: "small_links",
      theme: theme,
      data_type: ThemeSetting.types[:string],
      value: "some text, #|some text 2, #, invalid target",
    )

    run_theme_migration(theme, "0003-migrate-small-links-setting")

    expect(theme.settings[:small_links].value).to eq(
      [
        { "text" => "some text", "url" => "#", "target" => "_blank" },
        { "text" => "some text 2", "url" => "#", "target" => "_blank" },
      ],
    )
  end
end
```

This change is being introduced because we realised that writting just
javascript tests for the migrations is insufficient since javascript
tests do not ensure that the migrated theme settings can actually be
successfully saved into the database. Hence, we are introduce this
helper as a way for theme developers to write "end-to-end" migrations
tests.

app/services/theme_settings_migrations_runner.rb

@@ -53,7 +53,7 @@ class ThemeSettingsMigrationsRunner
     }
   JS
 
-  private_constant :Migration, :MIGRATION_ENTRY_POINT_JS
+  private_constant :MIGRATION_ENTRY_POINT_JS
 
   def self.loader_js_lib_content
     @loader_js_lib_content ||=
@@ -67,8 +67,8 @@ class ThemeSettingsMigrationsRunner
     @memory = memory
   end
 
-  def run
-    fields = lookup_pending_migrations_fields
+  def run(fields: nil, raise_error_on_out_of_sequence: true)
+    fields ||= lookup_pending_migrations_fields
 
     count = fields.count
     return [] if count == 0
@@ -80,12 +80,13 @@ class ThemeSettingsMigrationsRunner
 
     current_migration_version =
       @theme.theme_settings_migrations.order(version: :desc).pick(:version)
+
     current_migration_version ||= -Float::INFINITY
 
     current_settings = lookup_overriden_settings
 
     migrations.map do |migration|
-      if migration.version <= current_migration_version
+      if migration.version <= current_migration_version && raise_error_on_out_of_sequence
         raise_error(
           "themes.import_error.migrations.out_of_sequence",
           name: migration.original_name,
@@ -94,6 +95,7 @@ class ThemeSettingsMigrationsRunner
       end
 
       migrated_settings = execute(migration, current_settings)
+
       results = {
         version: migration.version,
         name: migration.name,
@@ -157,6 +159,7 @@ class ThemeSettingsMigrationsRunner
       .migration_fields
       .left_joins(:theme_settings_migration)
       .where(theme_settings_migration: { id: nil })
+      .order(created_at: :asc)
   end
 
   def convert_fields_to_migrations(fields)