CVE-2026-61681: Hatchet: SSRF via Unsigned UnsubscribeURL in SNS UnsubscribeConfirmation Handler

The SNS UnsubscribeConfirmation handler in internal/integrations/ingestors/sns/sns.go makes an unvalidated http.Get() call to payload.UnsubscribeURL without any URL restriction. Because UnsubscribeURL is intentionally excluded from the BuildSignature() signed field list, an attacker can replace this field in a legitimately AWS-signed message with an arbitrary internal URL, bypassing the VerifyPayload() signature check entirely and triggering a Server-Side Request Forgery (SSRF) against internal infrastructure — including the EC2 Instance Metadata Service (IMDS) at 169.254.169.254.