refactor(types): clarify 'validationContext' type with union

Co-authored-by: Manuel Spigolon <behemoth89@gmail.com>

fastify.d.ts

@@ -183,7 +183,7 @@ type TrustProxyFunction = (address: string, hop: number) => boolean
 declare module '@fastify/error' {
   interface FastifyError {
     validation?: ValidationResult[];
-    validationContext?: string;
+    validationContext?: "body" | "headers" | "parameters" | "querystring";
   }
 }