fix(wfe-yaml): replace SubWorkflow placeholder with real implementation

The YAML compiler was using SubWorkflowPlaceholderStep that returned
next() immediately. Replaced with real SubWorkflowStep from wfe-core
that starts child workflows and waits for completion events.

Added regression test verifying the compiled factory produces a step
that calls host_context.start_workflow() and returns wait_for_event.

wfe-yaml/tests/deno.rs

@@ -41,6 +41,7 @@ fn make_context<'a>(
         step,
         workflow,
         cancellation_token: tokio_util::sync::CancellationToken::new(),
+        host_context: None,
     }
 }