Correct payment_failed_url docs to reflect the deployed fallback behavior

Start with the Campaign Cart API guide and campaigns/v1.yaml, checking the create-order example, payment-failure callout, decline-flow explanation, and generated reference. Confirm the event-source fallback semantics with the implementation owner, then reconcile the guide and schema while keeping payment_failed_url optional; done means all public descriptions match the verified fallback order.

The Campaign Cart API guide says an omitted payment_failed_url is accepted only when the request includes HTTP Referer, and otherwise order creation fails. That describes superseded behavior. Engineering confirmed deployment of additional fallbacks on 2026-06-25; a read-only check of the current implementation on 2026-09-21 confirms the fallback resolver remains present.

Affected public surfaces:

• Campaign Cart API guide: the create-order example comment, “Send a Payment Failure URL” callout, and decline-flow explanation.
• Campaigns OpenAPI specification: CreateOrder.payment_failed_url has only a generic redirect description and does not explain fallback behavior.

Please reconcile the guide and schema with the current API behavior:

• Keep explicitly sending a suitable failure URL as the recommended integration practice.
• Remove the claim that omission of both the field and Referer necessarily rejects the order.
• Document the verified fallback order: explicit payment_failed_url, then Referer, then the campaign page URL header / available event-source fallback, then the origin of success_url. Confirm the precise event-source semantics with the implementation owner rather than describing it loosely as the shopper’s last page view.
• Keep the field optional in the OpenAPI schema; do not add the obsolete conditional requirement or make it unconditionally required.
• Check the example, callouts, and generated reference agree.

This is a documentation correction request, not a request to change API behavior. Historical documentation work: #44. This issue intentionally contains no merchant records or internal implementation excerpts.