fix: replace z.undefined() with z.never() for JSON Schema compat

Zod v4 cannot represent z.undefined() in JSON Schema, causing the
release workflow schema generation to fail. Replace with z.never()
in the AB test variant XOR union — semantically equivalent for
validation but serializable to JSON Schema.

Author: notgitika

src/schema/schemas/primitives/ab-test.ts

@@ -45,11 +45,11 @@ export type TargetRef = z.infer<typeof TargetRefSchema>;
 
 const ConfigBundleVariantConfigSchema = z.object({
   configurationBundle: ConfigurationBundleRefSchema,
-  target: z.undefined().optional(),
+  target: z.never().optional(),
 });
 
 const TargetVariantConfigSchema = z.object({
-  configurationBundle: z.undefined().optional(),
+  configurationBundle: z.never().optional(),
   target: TargetRefSchema,
 });