server: add --stop-idle-seconds for router mode

[leonardcser]

Summary

• Adds --stop-idle-seconds CLI flag that fully terminates idle model subprocesses in router mode after N seconds of inactivity
• On next request, ensure_model_loaded() re-spawns the process automatically (when --models-autoload is enabled)
• Unlike --sleep-idle-seconds which unloads VRAM/RAM within a child process, this flag kills the subprocess entirely

Refs #19379, follow-up to #18189

Test plan

• Build and verify --stop-idle-seconds appears in --help
• Run router mode with --stop-idle-seconds 5 --models-dir <dir>, send a request, wait >5s, verify process is terminated
• Send another request and verify the model re-spawns via autoload
• Verify existing router tests still pass

[ngxson]

I disagree with this feature, it sounds redundant to me.

The auto-sleep functionality added in #18228 should already allow model weight to be unloaded after a timeout, while also allowing static endpoints like /props, /models to be accessed without waking up the server. The current PR is missing that, and adding it will just duplicate a lot of code.

If the server instance need to be unloaded, it's up to the downstream application to decide when to unload it.

[leonardcser]

Thanks for the feedback

[apunkt]

If the server instance need to be unloaded, it's up to the downstream application to decide when to unload it.

agree, however unloading from downstream app now still leaves ps on gpu as mentioned in #19379_