ExtensionFixtures interface test

interface ExtensionFixtures {
  /**
   * Override built-in `context` with a persistent Chrome context that has
   * the Fill All extension loaded. MV3 service workers require a persistent
   * user data directory — regular browser contexts do NOT support them.
   */
  context: BrowserContext;
  /** Page created from the extension-enabled persistent context. */
  page: Page;
  /** The extension background service worker. */
  background: Worker;
  /** The extension ID extracted from the service worker URL. */
  extensionId: string;
  /** Auto fixture — collects JS coverage from chrome-extension:// URLs. */
  _coverage: void;
}