Fix update-path crashes, stale P2P state, and NCCL RDMA parsing

[morluto]

Summary

This fixes several failure paths in checkpoint update plumbing that can cause crashes, leaked resources, or incorrect RDMA device selection.

What was wrong

• load_metas() accepted empty owners, while gather_metas() filters them out. An empty owner can later reach bucket generation and hit an assertion on a zero-sized bucket.
• P2PStore.register_named_tensors() updated self.named_tensors before batch_register_memory() succeeded. If the engine registration failed, local state claimed tensors were registered even though the transfer engine rejected them.
• request_inference_to_update() created an httpx.Client for every update request without closing it, leaving the client transport/connection pool alive longer than necessary.
• NCCL_IB_HCA parsing did not fully match NCCL:
  • bare tokens should be prefix matches unless the value starts with =
  • empty exclude lists like ^ / ^= should select no devices, matching NCCL behavior

What changed

• Filter empty owners in load_metas() before storing global metadata.
• Update P2PStore.named_tensors only after transfer-engine registration succeeds.
• Use httpx.Client as a context manager in request_inference_to_update().
• Adjust RDMA parser behavior and tests to match NCCL semantics.

Review Notes

Suggested order:

1. checkpoint_engine/device_utils.py + tests/test_rdma_parser.py
2. checkpoint_engine/ps.py, checkpoint_engine/p2p_store.py, tests/test_p2p_store.py
3. checkpoint_engine/api.py + tests/test_api.py

Commits are split by area:

• fix: align RDMA device parsing with NCCL
• fix: keep parameter metadata state consistent
• fix: close inference update HTTP client

Testing

pytest tests/test_api.py tests/test_rdma_parser.py tests/test_assign_receiver_ranks.py tests/test_p2p_store.py -q

Result:

60 passed, 1 skipped

ruff check checkpoint_engine/device_utils.py checkpoint_engine/api.py checkpoint_engine/ps.py checkpoint_engine/p2p_store.py tests/test_rdma_parser.py tests/test_api.py tests/test_p2p_store.py
ruff format checkpoint_engine/device_utils.py checkpoint_engine/api.py checkpoint_engine/ps.py checkpoint_engine/p2p_store.py tests/test_rdma_parser.py tests/test_api.py tests/test_p2p_store.py --check

Both passed.