server : support multi-modal context checkpoints (#19849)

* Modify llama-memory-hybrid-iswa.cpp

* Modify llama-memory-recurrent.cpp

* Modify server-common.cpp

* Modify server-common.h

* Modify server-context.cpp

* Modify server-task.h

* Added comment to llama-memory-hybrid-iswa.cpp

* Remove comment from server-context.cpp

* Stylistic fix server-context.cpp

* Fix an issue when seqrm isn't called in server-context.cpp

* cont : alternative impl

* cont : cleanup

* cont : n_tokens -> int64_t

---------

Co-authored-by: timkhronos <timkhronos@gmail.com>

tools/server/server-task.h

@@ -557,6 +557,8 @@ struct server_prompt_checkpoint {
     llama_pos pos_min;
     llama_pos pos_max;
 
+    int64_t n_tokens;
+
     std::vector<uint8_t> data;
 
     size_t size() const {