test: add verification for HuggingFaceM4/InterleavedWebDocuments
Adds an integration test for the `HuggingFaceM4/InterleavedWebDocuments` dataset.

- Gracefully skips if the dataset is not yet available on the Hub
- Checks basic loading and structure once it becomes available

Closes #7394

First-time contributor to `datasets` — really excited about this! Happy to make any adjustments needed. 🙂