server: add SSE ping interval

[ngxson]

Overview

Supersede #23994

Refer to my investigation earendil-works/pi#5089 (comment) for more info

Add SSE ping (otherwise known as keepalive or comment)

Tested with:

• web UI client and python openai client, both correctly ignore the ping event
• nodejs's undici client: https://gist.github.com/ngxson/4556c9c598ba21aceda4f1fa6b540723 , no more timeout

Also remove the client-side message from #23842 , because this will (hopefully) make most clients happy now.

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: no

[aldehir]

Seems to work!

:

:

:

data: {"choices":[{"finish_reason":null,"index":0,"delta":{"role":"assistant","content":null}}],"created":1780390177,"id":"chatcmpl-uI7JNfldkYifvjYLehw5OnRid3v0JxBC","model":"Qwen3.6-27B-Q4_0.gguf","system_fingerprint":"b9465-23e6dab86","object":"chat.completion.chunk"}

data: {"choices":[{"finish_reason":null,"index":0,"delta":{"reasoning_content":"Here"}}],"created":1780390177,"id":"chatcmpl-uI7JNfldkYifvjYLehw5OnRid3v0JxBC","model":"Qwen3.6-27B-Q4_0.gguf","system_fingerprint":"b9465-23e6dab86","object":"chat.completion.chunk"}