test(form-core): expect prevValue in the multi-field linked listener spec

The target field does not change, so its listener receives its own previous value, an empty string, on both calls. This was the one watched-field assertion the earlier commits did not update, and it failed pnpm test:lib.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: galshir

packages/form-core/tests/FieldApi/listeners.spec.ts

@@ -664,11 +664,13 @@ describe('field - listeners', () => {
       expect(listener).toHaveBeenCalledTimes(2)
       expect(listener).toHaveBeenNthCalledWith(1, {
         value: '',
+        prevValue: '',
         fieldApi: targetField,
         formApi: form,
       })
       expect(listener).toHaveBeenNthCalledWith(2, {
         value: '',
+        prevValue: '',
         fieldApi: targetField,
         formApi: form,
       })