Add GPU placement validation before starting rollout engines

[fmh66]

Summary

Fixes #1896.

This PR adds an explicit GPU placement boundary check in ServerGroup.start_engines() before creating SGLang Ray actors.

Previously, when the placement group GPU slots and SGLang engine configuration were inconsistent, start_engines() could fail with an internal:

IndexError: list index out of range

at reordered_gpu_ids[gpu_index].

With this change, invalid configurations fail earlier with a ValueError that includes the relevant rollout configuration context.

Changes

• Add a lightweight validation helper for rollout server group GPU indices.
• Validate gpu_offset + num_engines * num_gpu_per_engine <= len(reordered_gpu_ids) before indexing placement group GPU IDs.
• Include actionable fields in the error message:
  • worker_type
  • gpu_offset
  • num_gpus_per_engine
  • num_engines
  • required_gpu_slots
  • len(reordered_gpu_ids)
  • rollout_num_gpus
  • rollout_num_gpus_per_engine
• Add unit coverage for valid configs, empty groups, and invalid configs with clear error context.

Testing

• python -m pytest tests/test_rollout_validation.py
• Manual validation of the helper behavior for both valid and invalid configurations.