feat: add va_list version of wrenCall and fix GC bug with return values

- Introduce wrenCallVarArgs as an explicit va_list variant, allowing variadic functions to forward their arguments directly without re-parsing.
- Fix a garbage collection issue in wrenCall where return values could be prematurely collected by ensuring proper root handling.
- Refactor the call API test to avoid re-entering the VM by moving test execution into an afterLoad callback instead of a foreign method.

src/cli/vm.c

@@ -12,6 +12,7 @@ static WrenVM* vm;
 
 static WrenBindForeignMethodFn bindMethodFn = NULL;
 static WrenBindForeignClassFn bindClassFn = NULL;
+static WrenForeignMethodFn afterLoadFn = NULL;
 
 static uv_loop_t* loop;
 
@@ -204,6 +205,8 @@ void runFile(const char* path)
 
   WrenInterpretResult result = wrenInterpret(vm, path, source);
 
+  if (afterLoadFn != NULL) afterLoadFn(vm);
+  
   if (result == WREN_RESULT_SUCCESS)
   {
     uv_run(loop, UV_RUN_DEFAULT);
@@ -260,8 +263,10 @@ uv_loop_t* getLoop()
 }
 
 void setTestCallbacks(WrenBindForeignMethodFn bindMethod,
-                      WrenBindForeignClassFn bindClass)
+                      WrenBindForeignClassFn bindClass,
+                      WrenForeignMethodFn afterLoad)
 {
   bindMethodFn = bindMethod;
   bindClassFn = bindClass;
+  afterLoadFn = afterLoad;
 }

src/cli/vm.h

@@ -23,6 +23,7 @@ uv_loop_t* getLoop();
 // Used by the API test executable to let it wire up its own foreign functions.
 // This must be called before calling [createVM()].
 void setTestCallbacks(WrenBindForeignMethodFn bindMethod,
-                      WrenBindForeignClassFn bindClass);
+                      WrenBindForeignClassFn bindClass,
+                      void (*afterLoad)(WrenVM* vm));
 
 #endif