fix(start): fix server function handling of standard schemas

Some implementations (namely Valibot) still include the value key for failure results, which is technically still correct in structural typing (since an object type is a *minimum* requirement). What can actually be relied on is that result.issues is undefined if the result is valid, as this is in the spec.

Currently failure results from Valibot are still treated as success.

packages/start/src/client/createServerFn.ts

@@ -17,8 +17,6 @@ import type {
   ResolveAllValidators,
 } from './createMiddleware'
 
-//
-
 export interface JsonResponse<TData> extends Response {
   json: () => Promise<TData>
 }
@@ -353,12 +351,13 @@ function execValidator(validator: AnyValidator, input: unknown): unknown {
   if ('~standard' in validator) {
     const result = validator['~standard'].validate(input)
 
-    if ('value' in result) return result.value
-
     if (result instanceof Promise)
       throw new Error('Async validation not supported')
 
-    throw new Error(JSON.stringify(result.issues, undefined, 2))
+    if (result.issues)
+      throw new Error(JSON.stringify(result.issues, undefined, 2))
+
+    return result.value
   }
 
   if ('parse' in validator) {