test(schemas): complete unresolved .resolves assertions in splitByTags test

Three `await expect(readFile(...)).resolves;` chains had no trailing
matcher, so the promise was awaited but no assertion ran. The file-
exists intent only surfaced via promise rejection. Added
`.toBeDefined()` to make the assertion explicit.

The broader readFile-vs-pathExists inconsistency is tracked in #3623.

Author: aqeelat

tests/api-generation.spec.ts

@@ -1450,9 +1450,9 @@ test('axios tags-split + schemas.splitByTags separates per-tag schemas from cros
   // Shared schemas referenced by both tags stay at the root, not in any tag
   // subdir. `Pagination` is shared even though it is only reached via
   // `PetList`/`StoreList` — both tag-scoped wrappers reference it.
-  await expect(readFile(path.join(root, 'error.ts'), 'utf8')).resolves;
-  await expect(readFile(path.join(root, 'sortOrder.ts'), 'utf8')).resolves;
-  await expect(readFile(path.join(root, 'pagination.ts'), 'utf8')).resolves;
+  await expect(readFile(path.join(root, 'error.ts'), 'utf8')).resolves.toBeDefined();
+  await expect(readFile(path.join(root, 'sortOrder.ts'), 'utf8')).resolves.toBeDefined();
+  await expect(readFile(path.join(root, 'pagination.ts'), 'utf8')).resolves.toBeDefined();
   // No tag owns a schema named after the other tag.
   await expect(
     readFile(path.join(root, 'pets', 'store.ts'), 'utf8'),