Render an email template without sending.
Args: render_request: Template rendering request with variables.
Returns: Rendered email content (subject, html_body, text_body).
Raises: HTTPException: 404 if template not found, 422 if rendering fails.
Request schema for rendering a template without sending.
Attributes: template_type: Template type to render (e.g., 'email_verification'). variables: Dictionary of variables for template rendering. locale: Optional language/locale code (default: 'en'). account_id: Optional account ID for template lookup.