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-helpers.cpp

@@ -310,6 +310,8 @@ std::vector<segment> prune_whitespace_segments(const std::vector<segment> & segm
 
 namespace autoparser {
 
+static const std::string ERR_TMPL = "#**ERROR**#";
+
 std::string apply_template(const common_chat_template & tmpl, const template_params & params) {
     generation_params tmpl_params;
     tmpl_params.messages              = params.messages;
@@ -326,7 +328,7 @@ std::string apply_template(const common_chat_template & tmpl, const template_par
         return common_chat_template_direct_apply(tmpl, tmpl_params);
     } catch (const std::exception & e) {
         LOG_DBG("Template application failed: %s\n", e.what());
-        return "";
+        return ERR_TMPL;
     }
 }
 
@@ -347,7 +349,7 @@ std::optional<compare_variants_result> compare_variants(
     std::string output_B = apply_template(tmpl, params_B);
 
     // Check for template application failures
-    if (output_A.empty() || output_B.empty()) {
+    if (output_A == ERR_TMPL || output_B == ERR_TMPL) {
         return std::nullopt;
     }