fix(architect): restore x-forwarded-host default handling for lambda servers

[SnowingFox]

Fixes #15378.

PR #15188 removed the useRequestContextDomainName option from the @react-router/architect adapter and made event.requestContext.domainName the sole default when deriving the request host, dropping x-forwarded-host from consideration entirely.

For Lambda Function URLs deployed behind CloudFront, requestContext.domainName is the internal *.lambda-url...on.aws host while the viewer-facing host is carried in the x-forwarded-host header populated by CloudFront. Because request.url was now built with the Lambda URL host, React Router's single-fetch throwIfPotentialCSRFAttack check compared the browser Origin (https://...cloudfront.net) against the Lambda URL and rejected legitimate same-origin browser POSTs.

This restores x-forwarded-host as the priority candidate when constructing the request URL, falling back to requestContext.domainName and then the Host header. That matches the pre-#15188 default behavior for proxied deployments while still preferring the request-context domain name when no proxy header is present.