FIX: Use theme screenshot names in theme fields (#31852)

Currently we allow for 2 theme screenshots to be specified,
with a lightweight spec to allow both a light and dark
version of the screenshot. However, we were not storing this
screenshot name anywhere, so we would not be able to use it
for light/dark switching.

This commit fixes that issue, and also does some general refactoring
around theme screenshots, and adds more tests.

lib/theme_screenshots_handler.rb

@@ -0,0 +1,83 @@
+# frozen_string_literal: true
+
+class ThemeScreenshotsHandler
+  MAX_THEME_SCREENSHOT_FILE_SIZE = 1.megabyte
+  MAX_THEME_SCREENSHOT_DIMENSIONS = [3840, 2160] # 4K resolution
+  MAX_THEME_SCREENSHOT_COUNT = 2
+  THEME_SCREENSHOT_ALLOWED_FILE_TYPES = %w[.jpg .jpeg .gif .png].freeze
+
+  class ThemeScreenshotError < StandardError
+  end
+
+  def initialize(theme)
+    @theme = theme
+  end
+
+  # Screenshots here come from RemoteTheme.extract_theme_info, which
+  # in turn parses the theme about.json file, which is where screenshots
+  # are defined.
+  def parse_screenshots_as_theme_fields!(screenshots, theme_importer)
+    updated_theme_fields = []
+    screenshots = Array.wrap(screenshots).take(MAX_THEME_SCREENSHOT_COUNT)
+    screenshots.each do |relative_path|
+      path = theme_importer.real_path(relative_path)
+      next if !path.present?
+
+      screenshot_filename = File.basename(path)
+      screenshot_extension = File.extname(path)
+
+      if !THEME_SCREENSHOT_ALLOWED_FILE_TYPES.include?(screenshot_extension)
+        raise ThemeScreenshotError,
+              I18n.t(
+                "themes.import_error.screenshot_invalid_type",
+                file_name: screenshot_filename,
+                accepted_formats: THEME_SCREENSHOT_ALLOWED_FILE_TYPES.join(","),
+              )
+      end
+
+      if File.size(path) > MAX_THEME_SCREENSHOT_FILE_SIZE
+        raise ThemeScreenshotError,
+              I18n.t(
+                "themes.import_error.screenshot_invalid_size",
+                file_name: screenshot_filename,
+                max_size:
+                  ActiveSupport::NumberHelper.number_to_human_size(MAX_THEME_SCREENSHOT_FILE_SIZE),
+              )
+      end
+
+      screenshot_width, screenshot_height = FastImage.size(path)
+      if (screenshot_width.nil? || screenshot_height.nil?) ||
+           screenshot_width > MAX_THEME_SCREENSHOT_DIMENSIONS[0] ||
+           screenshot_height > MAX_THEME_SCREENSHOT_DIMENSIONS[1]
+        raise ThemeScreenshotError,
+              I18n.t(
+                "themes.import_error.screenshot_invalid_dimensions",
+                file_name: screenshot_filename,
+                width: screenshot_width.to_i,
+                height: screenshot_height.to_i,
+                max_width: MAX_THEME_SCREENSHOT_DIMENSIONS[0],
+                max_height: MAX_THEME_SCREENSHOT_DIMENSIONS[1],
+              )
+      end
+
+      upload = RemoteTheme.create_upload(@theme, path, relative_path)
+      if !upload.errors.empty?
+        raise ThemeScreenshotError,
+              I18n.t(
+                "themes.import_error.screenshot",
+                errors: upload.errors.full_messages.join(","),
+              )
+      end
+
+      screenshot_filename_clean =
+        FileHelper.sanitize_filename(screenshot_filename.gsub(screenshot_extension, ""))
+      updated_theme_fields << @theme.set_field(
+        target: :common,
+        name: "screenshot_#{screenshot_filename_clean}",
+        type: :theme_screenshot_upload_var,
+        upload_id: upload.id,
+      )
+    end
+    updated_theme_fields
+  end
+end