Fix: JSI Value::toString() should check the result type (#59146)

Summary:
Pull Request resolved: #59146

`global.String` could be overridden. We always need to check the return type before casting to `jsi::String`.

Changelog: [Internal]

Reviewed By: avp

Differential Revision: D125707029

fbshipit-source-id: 15d3f83ac23d8bceb24e9e13a1f67dc4b61bb3b8

Author: lavenzg

packages/react-native/ReactCommon/jsi/jsi/jsi.cpp

@@ -923,7 +923,7 @@ String Value::asString(IRuntime& rt) && {
 
 String Value::toString(IRuntime& runtime) const {
   Function toString = runtime.global().getPropertyAsFunction(runtime, "String");
-  return toString.call(runtime, *this).getString(runtime);
+  return toString.call(runtime, *this).asString(runtime);
 }
 
 uint64_t BigInt::asUint64(IRuntime& runtime) const {