fix: preserve this when invoking standard validator (#14943)

The validate function call currently loses the dynamic this binding. Thus, if a standard validator's validate function is a class method that makes use of this, it will be undefined, as is the case when using TypeBox's standard schema adapter.

Author: thes01

.changeset/funny-seas-begin.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: preserve `this` when invoking standard validator

packages/kit/src/runtime/app/server/remote/shared.js

@@ -30,9 +30,8 @@ export function create_validator(validate_or_fn, maybe_fn) {
 		return async (arg) => {
 			// Get event before async validation to ensure it's available in server environments without AsyncLocalStorage, too
 			const { event, state } = get_request_store();
-			const validate = validate_or_fn['~standard'].validate;
-
-			const result = await validate(arg);
+			// access property and call method in one go to preserve potential this context
+			const result = await validate_or_fn['~standard'].validate(arg);
 
 			// if the `issues` field exists, the validation failed
 			if (result.issues) {