Cancel server work when clients disconnect

[mcollina]

Summary

• detect client FIN/RST while requests are queued or being processed
• detach abandoned queued/assigned jobs and cooperatively cancel prefill and decode work
• preserve in-flight decode and stack-owned job/socket lifetime during cancellation and shutdown
• prevent cancelled requests from publishing responses or stale live continuation state
• add no-model regressions for socket detection, queue ownership, progress cancellation, and decode accounting

A graceful peer FIN, including a write-side half-close, is intentionally treated as request cancellation for this one-request HTTP/1.1 server.

Validation

• make -j2 ds4_test ds4-server ds4_server_cpu.o
• ./ds4_test --server
• server-only test group repeated successfully
• git diff --check

A live model disconnect smoke test was not run; the existing server process was left running.

[mcollina]

Most harness have short timeouts and quick retries. This change avoid that a busy queue become a self-dos.