Run sandbox bank callback dry-run

Returns deterministic sandbox bank callback dry-run metadata when the authenticated automation client has banks.callbacks.manage. External URL: /v1/banks/current/callbacks. Current Next.js implementation path: /api/platform/v1/banks/current/callbacks. This sandbox-only route requires Idempotency-Key and validates callback signing/replay boundaries without delivering callbacks, calling providers, writing bank systems, creating applications, exposing callback signing secrets, exposing raw request/response bodies, exposing borrower contact data, or claiming API Gateway/mTLS readiness. Live bank callback delivery remains unavailable until promotion gates have REST, consent, audit, sandbox, mTLS-ready callback, operations, and enterprise-ingress evidence. Bank pilot posture remains information/referral and pre-approval only, recommendation and live bank-decision claims remain deferred, and partner-status publication remains private-first.