feat: Add merged vLLM rollout weights

[vivekkalyan]

Enable ART to serve merged LoRA weights through dedicated vLLM so Qwen3.5-MoE training works on the current vLLM build.

Changes

• Add rollout_weights_mode: "lora" | "merged" with dedicated-only validation and a merged-mode requirement for Qwen/Qwen3.5-35B-A3B and Qwen/Qwen3.5-397B-A17B
• Push merged weights into dedicated vLLM with native weight transfer while keeping LoRA checkpoints for training and persistence
• Update dedicated server wiring, validation, and Qwen3.5 smoke scripts/tests for the merged-inference path