fix: assorted bug fixes for CH migration

Fix resource leaks, race conditions, and error handling across host
agent and control plane: proper sparse file cleanup on close error,
connect error wrapping for MakeDir, CoW file cleanup on pause failure,
per-sandbox VM directories, deferred map deletion to avoid race in VM
destroy, and goroutine launch for extension background workers.

pkg/cpserver/run.go

@@ -255,7 +255,7 @@ func Run(opts ...Option) {
 	// Start extension background workers.
 	for _, ext := range o.extensions {
 		for _, worker := range ext.BackgroundWorkers(sctx) {
-			worker(ctx)
+			go worker(ctx)
 		}
 	}