Add actix-web seller example to validate actix integration

[Copilot]

Adds x402-kit/examples/actix_web_seller.rs to demonstrate actix-web paywall middleware patterns, mirroring the existing axum_seller.rs example.

Changes

• Standard paywall — handle_payment() flow via middleware::from_fn
• Custom paywall — Step-by-step API (process_request → settle → run_handler)
• Multi-payment paywall — Accepts both EVM and SVM payment schemes

Key integration pattern

Actix-web's ServiceRequest/ServiceResponse types are bridged to the paywall's HttpRequest/HttpResponse traits by splitting and reconstructing:

async fn standard_paywall(
    req: ServiceRequest,
    next: Next<impl MessageBody + 'static>,
) -> Result<ServiceResponse<impl MessageBody>, Error> {
    let (http_req, payload) = req.into_parts();
    let http_req_clone = http_req.clone();

    let response = paywall
        .handle_payment(http_req, |http_req| async move {
            let srv_req = ServiceRequest::from_parts(http_req, payload);
            match next.call(srv_req).await {
                Ok(res) => res.map_into_boxed_body().into_parts().1,
                Err(err) => err.error_response().map_into_boxed_body(),
            }
        })
        .await
        .map_err(Error::from)?;

    Ok(ServiceResponse::new(http_req_clone, response))
}

State is passed via app_data::<web::Data<AppState>>() and PaymentState is read from request extensions using the HttpMessage trait.

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