Create sandbox bank application dry-run

Creates deterministic sandbox bank application-intake dry-run metadata when the authenticated automation client has banks.applications.write. External URL: /v1/banks/current/applications. Current Next.js implementation path: /api/platform/v1/banks/current/applications. An exact eligible host grant may add a prepared action and first-party continuation for internal review; the API cannot confirm or reject it. This sandbox-only route requires Idempotency-Key, enable_platform_bank_frontends, and enable_platform_bank_workflow_write; it records replay-safe dry-run and prepared-action metadata only and never persists a bank application row, submits to a bank, calls providers, exposes bank secrets, exposes raw MyInfo, exposes credit bureau data, exposes bank statements, exposes raw document contents, exposes NRIC/FIN, exposes income-proof contents, sends callbacks, sends borrower notifications, returns signed storage URLs, or claims API Gateway/enterprise ingress readiness. Live bank application creation remains unavailable until promotion gates have REST, consent, audit, sandbox, document handoff, 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.