fix: resolve Zod JSON Schema generation failures

- Revert configBundles/abTests/httpGateways from .optional() back to
  .default([]) — Zod v4 cannot represent undefined in JSON Schema
- Replace z.undefined() with z.never() in AB test variant XOR union
- Strip empty evo arrays in writeProjectSpec so they don't appear in
  agentcore.json for users who haven't opted into preview features

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,
 });