fix(code-runtime-python): bind str for dispatch's rejection message conversion

The review's remaining non-blocking suggestion: dispatch's call_failure(str(exc))
resolved the builtin str at call time, so a program rebinding __main__.str could
run a hostile callable when the binding-rejection message is formatted. Bind
_str into _run locals and use it in dispatch.

packages/code-runtime/code-runtime-python/tests/runtime.spec.ts

@@ -690,7 +690,7 @@ describe('PythonCodeRuntime — programs and bindings', () => {
         'try:',
         '    await tools.fail({})',
         'except RuntimeError as e:',
-        '    caught = str(e)',
+        '    caught = e.args[0] if e.args else ""',
         'except Exception as e:',
         '    caught = "WRONG TYPE: " + type(e).__name__',
         'return caught',