server: fix checkpoints creation (#22929)

* common : add common_chat_split_by_role

* cont : fix spans to reach end of message

* server: fix checkpoints creation

- extract message_spans from chat templates
- find the prompt token position before the latest user message
- split prompt batching at that position
- create a context checkpoint before the latest user input
- avoid periodic mid-prompt checkpoints when that position is known
- handle multimodal prompts when mapping text/template positions to server prompt tokens
- add --checkpoint-min-step to control minimum spacing between checkpoints

* cont : clean-up

* Support autoparser detection for message barriers

* server: fix message span delimiter and update docs

---------

Co-authored-by: Alde Rojas <hello@alde.dev>
Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>
Co-authored-by: Piotr Wilkin <piotr.wilkin@syndatis.com>

common/chat.h

@@ -143,6 +143,17 @@ struct common_chat_msg_diff {
     }
 };
 
+struct common_chat_msg_span {
+    std::string role;
+    std::size_t pos = 0;
+    std::size_t len = 0;
+};
+
+struct common_chat_msg_delimiter {
+    std::string role;
+    std::string delimiter;
+};
+
 struct common_chat_tool {
     std::string name;
     std::string description;
@@ -208,6 +219,7 @@ struct common_chat_params {
     std::vector<std::string>            preserved_tokens;
     std::vector<std::string>            additional_stops;
     std::string                         parser;
+    std::vector<common_chat_msg_span>   message_spans;
 };
 
 // per-message parsing syntax
@@ -304,6 +316,7 @@ std::optional<common_chat_params> common_chat_try_specialized_template(
         const std::string &                   src,
         autoparser::generation_params & params);
 
+
 // specialized per-task preset
 struct common_chat_prompt_preset {
     std::string system;
@@ -311,3 +324,6 @@ struct common_chat_prompt_preset {
 };
 
 common_chat_prompt_preset common_chat_get_asr_prompt(const common_chat_templates * chat_templates);
+
+std::vector<common_chat_msg_span> common_chat_split_by_role(const std::string & prompt, const std::vector<common_chat_msg_delimiter> & delims);
+