The provider mints the session id returned on AgentSession. Creation is
idempotent on idempotency_key scoped per caller (context.subject.id):
a replayed key returns the existing session, an empty key always creates.
Idempotency is scoped to the provider's session store.
The provider mints the session id returned on AgentSession. Creation is idempotent on idempotency_key scoped per caller (context.subject.id): a replayed key returns the existing session, an empty key always creates. Idempotency is scoped to the provider's session store.