Add routed_expert_logits support to R3 datums

[kiddyboots216]

Summary

• add optional routed_expert_logits support to xorl_client.types.Datum
• preserve and lift both R3 routing fields during TrainingClient datum conversion so request payloads include routed_experts and routed_expert_logits
• update chunking tests to follow MAX_CHUNK_LEN so they exercise the current multi-chunk behavior consistently
• add tests covering datum serialization and forward_backward request construction for both routing fields

Closes #4.

Testing

• python -m pytest -q /home/apanda/xorl-client-issue-4/tests/test_types.py /home/apanda/xorl-client-issue-4/tests/test_chunked_forward_backward.py

[kiddyboots216]

Folded into #2. The examples/filler-rl-pipeline branch now contains the routed_expert_logits changes plus the GSM8K/pipeline and worker_port fixes, so I am closing this standalone PR as superseded.