Download the instance serial console log as a plain-text file

Streams the full serial console log (ttyS0 mirror) of the selected instance as a plain-text file, as triggered by the "Download log" button on the serial log tab of the instance console page. Each line carries the kernel-clock timestamp and message exactly as rendered in the console ([ 0.000000] Linux version …), covering boot, systemd, cloud-init and agent output. The suggested filename is <guest-hostname>-ttyS0.log via Content-Disposition. The serial log remains reachable even when the guest network is down, which is what makes this export useful for boot/fstab/firewall-lockout debugging. Validation: scopeId must be accessible to the caller (404 via ErrNotFound when unknown); instanceId must identify an instance in that scope (404 NOT_FOUND); the instance must have a serial console buffer — instances without any captured output return an empty file rather than an error. No body or query inputs beyond the path. Async & state: Synchronous read; no state change. The response is a point-in-time snapshot of the buffer; live updates continue to flow only through the console session stream, not this endpoint. Errors: 401 unauthenticated; 403 caller lacks console access on the scope (owner|admin|contributor); 404 unknown scopeId or instanceId; 500 on log retrieval failure. Idempotency/Concurrency: Safe, cacheable; no idempotency key required. Repeated downloads may differ only by lines appended between requests.