Run one step as the caller in context. The step's tool is the instance
the pipeline resolved and checked, so a registration that replaces the
name between the check and the run is not what runs. A signal that is
already aborted when the checks are done stops the step before its tool.
Optional signal: AbortSignal
Runs one step with the host's checks. Built by
ToolOrchestrator, or by a host with createStepGate.