Cancels a running task. The implementation should stop execution and
publish a final canceled status event on the provided bus.
Executes the agent logic and publishes events to the bus.
Every call MUST publish either a task or a message event as its
first event — including follow-up turns where requestContext.task
is already set. The server enforces this ordering and rejects
streams that begin with a statusUpdate or artifactUpdate.
Multi-tenant implementations can read the tenant identifier from
requestContext.context.tenant.
Optional remote-agent seam. It discovers an official v1 AgentCard, selects JSON-RPC only, and maps remote lifecycle IDs onto the local task boundary.