Resolve Client.Go-001 code-review finding

MxAccessError.Unwrap returned e.Command directly; on the HRESULT-only path
Command is a nil *CommandError, so Unwrap returned a non-nil error wrapping
a typed nil and errors.As bound a nil *CommandError. Unwrap now returns an
untyped nil when Command is nil. Added errors_test.go regression coverage
for the HRESULT-only and populated-Command paths.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

clients/go/mxgateway/errors.go

@@ -85,8 +85,12 @@ func (e *MxAccessError) Error() string {
 }
 
 // Unwrap returns the wrapped CommandError, when one is present.
+//
+// When Command is nil (the HRESULT / MxStatusProxy path) it returns an
+// untyped nil rather than a typed-nil *CommandError, so errors.As does not
+// bind a nil pointer that a caller would then panic on.
 func (e *MxAccessError) Unwrap() error {
-	if e == nil {
+	if e == nil || e.Command == nil {
 		return nil
 	}
 	return e.Command