Get correct NodeManagerScope for toStrings in API (#5216)

Get correct NodeManagerScope for toStrings in API (#5216)

Gets the correct `NodeManagerScope` for getting the `toString` of a term, op, or sort. The following program had strange output:
```
#include "cvc4/api/cvc4cpp.h"
using namespace CVC4::api;
using namespace std;
int main()
{
  Solver s;
  Term x = s.mkConst(s.getIntegerSort(), "x");
  cout << "x = " << x << endl;
  Solver s2;
  cout << "x = " << x << endl;
  return 0;
}
```

It was outputting:
```
x = x
x = var_267
```

Fixing the scope makes the output `x = x` both times, as expected.