server: use pos_next instead of n_tokens for m-rope (#22439)

Author: am17an

tools/server/server-context.cpp

@@ -3031,7 +3031,7 @@ struct server_context_impl {
                 slot.sampled = ids.back(); // last accepted token
                 SLT_DBG(slot, "add accepted tokens: sampled=%d, ids.size=%zu, n_draft=%zu\n", slot.sampled, ids.size(), n_draft);
 
-                llama_memory_seq_rm(llama_get_memory(slot.ctx), slot.id, slot.prompt.n_tokens(), -1);
+                llama_memory_seq_rm(llama_get_memory(slot.ctx), slot.id, slot.prompt.tokens.pos_next(), -1);
 
                 for (size_t i = 0; i < ids.size(); ++i) {
                     completion_token_output result;