Unify ray serve

[lbluque]

Summary

• Unified batch serving around BatchPredictServer + BatchServerPredictUnit, removing the parallel FAIRChemInferenceServer / FAIRChemInferenceClient / RayServeMLIPUnit architecture (~1,100 lines deleted across 3 files)
• Added MultiplexedBatchPredictServer — a subclass of BatchPredictServer that uses @serve.multiplexed for on-demand model loading with LRU eviction, preserving the multi-model capability in a cleaner form
• Added BatchServerPredictUnit.from_deployment_connection_info() classmethod to connect to already-running Ray Serve deployments by name, with optional multiplexed_model_id for multiplexed deployments — replacing the need for RayServeMLIPUnit
• Moved wait_for_serve_ready() and get_ray_connection_info() into batch_predict_server.py as shared utilities
• Extracted _init_ray_and_serve() and _build_deployment_options() helpers to share setup logic between setup_batch_predict_server() and the new setup_multiplexed_batch_predict_server()
• Updated get_slurm_ray_cluster and get_local_ray_cluster to accept a predict_unit parameter and use setup_batch_predict_server() instead of the deleted start_serve()

What's removed

• FAIRChemInferenceServer / FAIRChemInferenceClient / RayServeMLIPUnit: 3 files deleted entirely; their functionality is replaced by MultiplexedBatchPredictServer + BatchServerPredictUnit.from_deployment_connection_info()
• Metadata serialization layer (RayServeTask, _cache_model_metadata, fetch_model_metadata): replaced by get_predict_unit_attribute which returns real objects via Ray serialization
• SimpleNamespace inference_settings stub: replaced by fetching real InferenceSettings from the server

Test plan

• pytest tests/core/calculate/test_batcher.py -c packages/fairchem-core/pyproject.toml -vv
• pytest tests/core/units/mlip_unit/test_predict.py -c packages/fairchem-core/pyproject.toml -vv
• pytest tests/core/units/mlip_unit/test_inference_serve.py -c packages/fairchem-core/pyproject.toml -vv (requires GPU — covers both single-model and multiplexed server tests)