toolChoice property
Spec 2025-11-25+: tool-selection directive. Carried losslessly — the
wire value is either a mode string ("auto" / "none" / "required")
or an object naming a specific tool (e.g. {"type":"tool","name":"…"}).
Modeled as Object rather than a rigid enum so any peer-defined shape
round-trips without loss; use SamplingToolChoice helpers to build it.
Implementation
final Object? toolChoice;