server: improve user message detection and create checkpoints at every user message (#24176)

* server : improve message span logic

* cont : cast size_t to int32_t in comparisons

* server : create checkpoints before every user msg

* chat : remove \n in gemma4 delimiters

* chat : merge msg delimiter structs into one

* cont : reword comment

* cont : initialize tokens in delimiter

* cont : add server_tokens::get_raw_tokens() for mtmd

* cont : move message finding to server_tokens and skip mtmd tokens

* cont : update cohere2moe parser

* cont : increase min-step to 8192 and always produce a chkpt for last user message

common/common.h

@@ -609,7 +609,7 @@ struct common_params {
     bool    cache_prompt        = true;  // whether to enable prompt caching
     bool    cache_idle_slots    = true;  // save and clear idle slots upon starting a new task
     int32_t n_ctx_checkpoints   = 32;    // max number of context checkpoints per slot
-    int32_t checkpoint_min_step = 256;   // minimum spacing between context checkpoints
+    int32_t checkpoint_min_step = 8192;  // minimum spacing between context checkpoints
     int32_t cache_ram_mib       = 8192;  // -1 = no limit, 0 - disable, 1 = 1 MiB, etc.
 
     std::string hostname      = "127.0.0.1";