vllm: accept HF repo ids (not just local dirs) in model resolution

_resolve_model_dir now returns a model-list entry's path even when it's a HF repo id
(e.g. Qwen/Qwen2.5-0.5B-Instruct), and treats an org/model model_name as a repo id.
vLLM's --model accepts both; the previous isdir-only check returned None -> 'no model
resolved'.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Mw2KQiswmD69T45fTfjKwW