fix: Fix dataset snapshots silently including deleted rows

[Luca Forstner (lforst)]

Closes braintrustdata/braintrust-sdk-javascript#2492

• Fix dataset snapshots including deleted rows.
• Previously, snapshots used the newest transaction from surviving rows, which could point to a moment before the deletions.
• After uploading pending changes, request a current transaction ID from the server so snapshots include deletions even when every row has been deleted. Datasets pinned to a specific historical version keep that version.