server: avoid unnecessary checkpoint restore when new tokens are present (#24110)

• server: avoid unnecessary checkpoint restore when new tokens are present

The pos_min_thold calculation unconditionally subtracts 1 to ensure at least one token is evaluated for logits when no new tokens exist. However, when the request contains new tokens beyond the cached prefix, this -1 is overly conservative and may trigger an unnecessary checkpoint restore.

Conditionally apply the -1 only when n_past >= task.n_tokens() (no new tokens), avoiding redundant KV state restoration when there is actual work to do.

• cont : add ref

Co-authored-by: Georgi Gerganov [email protected]