b6989

server: fix correct time_ms calculation in prompt_progress (#17093)

* fix: correct time_ms calculation in send_partial_response

The time_ms field was incorrectly calculated. The division was happening
before the subtraction leading to incorrect values.

Before: (ggml_time_us() - slot.t_start_process_prompt / 1000) After:
(ggml_time_us() - slot.t_start_process_prompt) / 1000

* docs : document time_ms field in prompt_progress