server : [easy] fix per round speculative decode logging (#18211)

Currently we always log 0, as we clear slot.drafted before.

To reproduce:
Run llama-server with devstral-2 as main model and devstral-2-small as
md, and verbose logging:

```
% ./build/bin/llama-server -v  \
  -m ~/llms/Devstral-2-123B-Instruct-2512-UD-Q6_K_XL-00001-of-00003.gguf \
  -md ~/llms/Devstral-Small-2-24B-Instruct-2512-UD-Q2_K_XL.gguf \
  -c 8192 2> /tmp/llama.cpp.debug

Check the log:

slot update_slots: id  3 | task 0 | accepted 11/0 draft tokens, new
n_tokens = 741
slot update_slots: id  3 | task 0 | accepted 4/0 draft tokens, new
n_tokens = 746
slot update_slots: id  3 | task 0 | accepted 16/0 draft tokens, new
n_tokens = 763
slot update_slots: id  3 | task 0 | accepted 11/0 draft tokens, new
n_tokens = 775
slot update_slots: id  3 | task 0 | accepted 2/0 draft tokens, new
n_tokens = 778
slot update_slots: id  3 | task 0 | accepted 4/0 draft tokens, new
n_tokens = 783
slot update_slots: id  3 | task 0 | accepted 8/0 draft tokens, new
n_tokens = 792
slot update_slots: id  3 | task 0 | accepted 2/0 draft tokens, new
n_tokens = 795
slot update_slots: id  3 | task 0 | accepted 1/0 draft tokens, new
n_tokens = 797
slot update_slots: id  3 | task 0 | accepted 1/0 draft tokens, new
n_tokens = 799
slot update_slots: id  3 | task 0 | accepted 0/0 draft tokens, new
n_tokens = 800
slot update_slots: id  3 | task 0 | accepted 2/0 draft tokens, new
n_tokens = 803
slot update_slots: id  3 | task 0 | accepted 1/0 draft tokens, new
n_tokens = 805
slot update_slots: id  3 | task 0 | accepted 6/0 draft tokens, new
n_tokens = 812
slot update_slots: id  3 | task 0 | accepted 3/0 draft tokens, new
n_tokens = 816
```

After the fix, get correct per round logging:

```
slot update_slots: id  3 | task 0 | accepted 7/8 draft tokens, new
n_tokens = 654
slot update_slots: id  3 | task 0 | accepted 1/2 draft tokens, new
n_tokens = 656
slot update_slots: id  3 | task 0 | accepted 2/16 draft tokens, new
n_tokens = 659
slot update_slots: id  3 | task 0 | accepted 1/16 draft tokens, new
n_tokens = 661
slot update_slots: id  3 | task 0 | accepted 2/16 draft tokens, new
n_tokens = 664
slot update_slots: id  3 | task 0 | accepted 16/16 draft tokens, new
n_tokens = 681
slot update_slots: id  3 | task 0 | accepted 16/16 draft tokens, new
n_tokens = 698
slot update_slots: id  3 | task 0 | accepted 3/4 draft tokens, new
n_tokens = 702
slot update_slots: id  3 | task 0 | accepted 5/12 draft tokens, new
n_tokens = 708
slot update_slots: id  3 | task 0 | accepted 16/16 draft tokens, new
n_tokens = 725
slot update_slots: id  3 | task 0 | accepted 1/1 draft tokens, new
n_tokens = 727
slot update_slots: id  3 | task 0 | accepted 8/16 draft tokens, new
n_tokens = 736
```

tools/server/server-context.cpp

@@ -2628,7 +2628,7 @@ struct server_context_impl {
                     }
                 }
 
-                SLT_DBG(slot, "accepted %d/%d draft tokens, new n_tokens = %d\n", (int) ids.size() - 1, (int) slot.drafted.size(), slot.prompt.n_tokens());
+                SLT_DBG(slot, "accepted %d/%d draft tokens, new n_tokens = %d\n", (int) ids.size() - 1, (int) n_draft, slot.prompt.n_tokens());
             }
         }