cancel method
Cancel the in-flight native loop. rac_tool_calling_session_cancel_proto
is thread-safe and idempotent, so it is invoked directly from this isolate
(it latches a cancel the worker's in-flight generate checks) rather than
queued behind the worker's blocking turn.
Implementation
bool cancel() {
if (_closed || _resolvedId == 0) return false;
return DartBridgeToolCalling.shared.cancelSession(_resolvedId);
}