Refresh temporary namespace credentials in distributed workers

[FANNG1]

Goal

Allow long-running distributed namespace operations to refresh temporary catalog-vended storage credentials.

Scope

• Introduce an explicit serializable table-access context instead of relying on private LanceDataset attributes.
• Recreate the namespace client and reopen datasets on distributed workers where needed.
• Cover scan, merge, index, and compaction workers.
• Add a short-lived credential or forced-refresh regression test.

This is a follow-up reliability optimization and does not block the initial namespace feature, release, or Daft entry-point integration tracked by #51.