Mark sandbox Call() method virtual

This is to enable custom error handling across all sandbox method calls.

PiperOrigin-RevId: 684825280
Change-Id: I8b6eaa1b9adef71ee72666af8083c979e25bdd77

sandboxed_api/sandbox.h

@@ -113,8 +113,8 @@ class Sandbox {
  "Too many arguments to sapi::Sandbox::Call()");
  return Call(func, ret, {std::forward<Args>(args)...});
  }
- absl::Status Call(const std::string& func, v::Callable* ret,
- std::initializer_list<v::Callable*> args);
+ virtual absl::Status Call(const std::string& func, v::Callable* ret,
+  std::initializer_list<v::Callable*> args);
 
  // Allocates memory in the sandboxee, automatic_free indicates whether the
  // memory should be freed on the remote side when the 'var' goes out of scope.