[Feature request] Support deterministic sampling for `GSgnnLinkPredictionTestDataLoader`

Validation/test sets for link-prediction tasks should yield the same set of negative links to make the metrics comparable across experiments on the same graph.

Locate the GSgnnLinkPredictionTestDataLoader entry point and read how validation and test negative links are currently sampled. Determine how a deterministic sampling option should be exposed and verify that repeated runs on the same graph produce identical negative-link sets and comparable metrics.

Written by the indexing model from the issue text.