test: add dataloader checkpoint integration test for retrieval recipes

[oliverholworthy]

Summary

• Adds integration test validating that bi-encoder and cross-encoder StatefulDataLoaders correctly save and restore iteration state across checkpoint boundaries
• Ensures resumed training skips already-seen samples by verifying batch-level continuity after checkpoint restore
• Follows the same pattern as test_megatron_dataset_checkpoint.py but targets the retrieval build_dataloader path

Test plan

• test_bi_encoder_dataloader_checkpoint — passes locally with torchrun --nproc-per-node=1
• test_cross_encoder_dataloader_checkpoint — passes locally with torchrun --nproc-per-node=1
• CI validation

Run locally:

HF_HOME="" uv run torchrun --nproc-per-node=1 -m pytest \
    tests/functional_tests/training/test_retrieval_dataloader_checkpoint.py -vs \
    --override-ini="addopts="