fix(python): check each parameter in native callback validation loop

[jdymitarai]

Summary

In python/_jsonnet.c, handle_native_callbacks validates that each parameter specified in a native callback's parameter tuple is a string:

        /* Check the params are all strings */
        num_params = PyTuple_Size(params);
        for (i = 0; i < num_params ; ++i) {
            PyObject *param = PyTuple_GetItem(params, 0);
            if (!PyUnicode_Check(param)) {
                PyErr_SetString(PyExc_TypeError, "native callback param must be string");
                goto bad;
            }
        }

The loop was indexing params at index 0 instead of loop counter i. As a consequence:

• If a callback had multiple parameters where parameter 0 was a string but any subsequent parameter was not (e.g. (('name', 123), cb)), the loop repeatedly validated parameter 0 and completely skipped subsequent parameters.
• In the subsequent setup loop, get_py_utf8_string called PyUnicode_AsUTF8String on the non-string object, which set an uncaught exception on the CPython thread state and produced a NULL string pointer.
• evaluate_snippet then returned a result with an active exception, causing CPython to raise:
  SystemError: <built-in function evaluate_snippet> returned a result with an exception set

Changes

• Fix parameter indexing from PyTuple_GetItem(params, 0) to PyTuple_GetItem(params, i).
• Add unit test test_native_callback_param_type_validation in python/_jsonnet_test.py.