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-auto-parser.h

@@ -377,6 +377,8 @@ struct analyze_tools : analyze_base {
 
 struct autoparser {
     jinja::caps          jinja_caps;
+    std::string          user_start;
+    std::string          assistant_start;
     analyze_reasoning    reasoning;
     analyze_content      content;
     analyze_tools        tools;
@@ -387,6 +389,10 @@ struct autoparser {
 
     autoparser() = default;
 
+    // Find the starting marker for the user message and assistant message
+    std::string detect_user_start_marker(const common_chat_template & tmpl);
+    std::string detect_assistant_start_marker(const common_chat_template & tmpl);
+
     // Run full differential analysis on a template
     void analyze_template(const common_chat_template & tmpl);