Address second round of PR review comments

- Close async iterators in ModelHolder after reading first value to
  prevent resource leaks
- Catch exceptions from validateMetaSchema in Zod refine predicate
  so validation errors flow through Zod's normal issue reporting
- Warn on duplicate catalog model layer IDs instead of silently
  dropping later entries
- Replace `as any` with `as JsonObject` for schema import in
  scaffolder template model layer

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Fredrik Adelöw <freben@spotify.com>

plugins/scaffolder-common/src/catalogModel.ts

@@ -15,6 +15,7 @@
  */
 
 import { createCatalogModelLayer } from '@backstage/catalog-model/alpha';
+import { JsonObject } from '@backstage/types';
 import schema from './Template.v1beta3.schema.json';
 
 /**
@@ -47,7 +48,7 @@ export const templateModelLayer = createCatalogModelLayer({
             },
           ],
           schema: {
-            jsonSchema: schema as any,
+            jsonSchema: schema as JsonObject,
           },
         },
       ],