fix: remove redundant "Receiver" prefix from method-not-found error message

The error message for unimplemented methods previously included the word "Receiver"
before the class name, which was redundant and inconsistent with other error messages.
Updated the format string in `methodNotFound()` to drop "Receiver" and use the class
name directly. Added a new test case for static method not found errors and updated
existing test expectations to match the new message format.

src/wren_vm.c

@@ -328,7 +328,7 @@ static void methodNotFound(WrenVM* vm, ObjFiber* fiber, int symbol, ObjClass* cl
 {
   char message[100];
 
-  snprintf(message, 100, "Receiver '%s' does not implement method '%s'.",
+  snprintf(message, 100, "%s does not implement method '%s'.",
            classObj->name->value,
            vm->methods.names.data[symbol]);