reCAPTCHA Enterprise site key from the callback payload.
Use rn-recaptcha (verifyForJourney) to drive the verification flow — the
native callback reads the site key from its own server payload and
auto-submits the token into the callback inputs.
JourneyReCaptchaEnterpriseField.siteKey is informational only (for example, rendering a "protected by reCAPTCHA" badge) and is never needed to execute verification.
Normalized field for a
ReCaptchaEnterpriseCallback.