Fix flaky test ChunkLinkDownloadServiceTest#testBatchDownloadChaining

[jayantsing-db]

Description

The link download futures were being completed because the link download chain was triggered eagerly. When getLinkForChunk is called, the link download future is evaluated. If the future is already complete, it checks the status of the chunk. The test was not mocking the chunk, which caused a NullPointerException (NPE). Locally, the test passed because the link download futures weren't completing before getLinkForChunk was called. However, in the GitHub runs, the futures were completing, which led to a thread synchronization issue.

NO_CHANGELOG=true

Testing

Additional Notes to the Reviewer

[github-actions]

Please ensure that the `NEXT_CHANGELOG.md` file is updated with any relevant changes.  
If this is not necessary for your PR, include this in the PR body:

```
NO_CHANGELOG=true
```

and rerun the workflow.