Type Alias ToolExecuteFn 

pub type ToolExecuteFn = extern "C" fn(tool_call_id: StbStringRef, params: StbString, free_params: Option<FreeStringFn>) -> StepHandle;

execute(tool_call_id, params) -> StepHandle. Plugin-allocates a drive handle and begins the work (non-blocking — the real progress comes via poll). tool_call_id is a borrowed StbStringRef (valid for the call); params is an owning JSON string of the tool-call arguments (the plugin frees it via the host’s free_string). Returns null on allocation failure.