Send a standard A2A JSON-RPC request

A2A 1.0 JSON-RPC accepts SendMessage, SendStreamingMessage, GetTask, CancelTask, and SubscribeToTask. A2A 0.3 accepts message/send, message/stream, tasks/get, tasks/cancel, and tasks/resubscribe. Streaming methods return server-sent events. For SendMessage, send an advertised skill id as params.metadata.skill; an omitted skill is restricted to general discovery. Requires 'discovery' scope for unhinted requests and discovery skills, 'creator' scope for matching, audience mutation and delivery skills, or 'account' scope for read-only account-info. Task reads and cancellation are restricted to the owning organization. Credit-purchase, portal and organization-administration skills are unavailable on public A2A.