refactor(srt): take the GB300 DSv4 Mooncake RDMA devices from the fabric

prefill role. Like the other recipes' NIC lists, both read '@fabric.rdma-devices',
which renders the same mlx5_0..mlx5_3 list on gb300-nv.

Author: adibarra

inferencex-e2e/benchmarks/multi_node/srt-slurm-recipes/dsv4/sglang/gb300-fp4/agentx/disagg-variants.yaml

@@ -51,7 +51,7 @@ base:
       args: ["--port", "8800"]
       env:
         MOONCAKE_PROTOCOL: rdma
-        MOONCAKE_DEVICE: "mlx5_0,mlx5_1,mlx5_2,mlx5_3"
+        MOONCAKE_DEVICE: "@fabric.rdma-devices"
         MOONCAKE_GLOBAL_SEGMENT_SIZE: 180gb
         MC_STORE_CLIENT_METRIC_INTERVAL: '5'
         MC_ENABLE_DEST_DEVICE_AFFINITY: '1'
@@ -137,7 +137,7 @@ base:
         MOONCAKE_PROTOCOL: rdma
         # The store only registers host buffers on RDMA; without an explicit list it
         # falls back to NVLink for same-domain peers and aborts at the warmup put.
-        MOONCAKE_DEVICE: "mlx5_0,mlx5_1,mlx5_2,mlx5_3"
+        MOONCAKE_DEVICE: "@fabric.rdma-devices"
         MOONCAKE_GLOBAL_SEGMENT_SIZE: '@dram.per-gpu-bytes'
         MOONCAKE_STANDALONE_STORAGE: '0'
         MC_STORE_CLIENT_METRIC_INTERVAL: '5'