Get connection descriptors for the instance

Produces the connection descriptors shown by the Connect tab of the instance detail page: protocol (SSH for Linux images, RDP for Windows), target host (floating IP preferred, else fixed IP), port (22 or 3389), the OS-derived username (Administrator / ubuntu / admin / cloud-user), the associated key-pair name, and a ready-to-paste client command (ssh -i … or mstsc /v:…). No resource state is mutated; the call exists so the API can resolve credentials/host data server-side and audit connection intents. Validation: instance must exist and be visible → 404 NOT_FOUND; instance must be ACTIVE — the UI warns "start it before connecting" and the backend rejects non-running instances → 409 CONFLICT; the instance must have at least one reachable IP (fixed or floating), otherwise the host is returned as null and the command omitted. Async & state: Synchronous (200) returning InstanceConnectionInfo; no instance state change. No SSE frames are emitted for this action. Errors: 401 UNAUTHENTICATED; 403 FORBIDDEN when the caller may not connect to resources in this scope; 404 NOT_FOUND for an unknown instanceId; 409 CONFLICT when the instance is not in a connectable (ACTIVE) state; 500 INTERNAL. Idempotency/Concurrency: Effectively read-like but issued as a POST for auditability; this operation does not use Idempotency-Key (it may be omitted and a sent one is ignored). Each call recomputes the descriptor from the instance's current state, so repeated calls return the same payload while the instance is unchanged. No If-Match applies since nothing is mutated.