server: clear checkpoints on context shift to prevent stale restore faults

After context shift, slot.prompt.checkpoints retained pre-shift positions.
Subsequent checkpoint restore via llama_state_seq_set_data_ext used stale
positions causing GPU memory access faults on multi-GPU ROCm setups.

Author: kmbandy

tools/server/server-context.cpp

@@ -2257,6 +2257,7 @@ struct server_context_impl {
                     slot.prompt.tokens.insert(new_tokens);
                 }
 
+                slot.prompt.checkpoints.clear();
                 slot.truncated = true;
             }
         }