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 [email protected] Co-authored-by: Georgi Gerganov [email protected] Co-authored-by: Piotr Wilkin [email protected]