Drive the rollout server cells from reconcile against the worker provider

[fzyzcjy]

Part of #1837

[gemini-code-assist]

Caution

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[guapisolo]

[P2] Reject cross-node TP widths that do not tile the node

With num_gpus=24, num_gpus_per_engine=12, and num_gpus_per_node=8, this validation accepts the group and builds two one-worker/8-slot cells while each engine is launched with tp_size=12 and nnodes=1. Each worker therefore exposes only 8 GPUs to a 12-rank engine, so startup can fail to become healthy and wait indefinitely. No later PR through checkpoint #2176 or checked tip #2649 repairs this gap. Please fail fast unless partial-node workers are explicitly supported.