CJS http.ServerResponse subclass (ServerResponse.call + util.inherits): assignSocket is a no-op (breaks fastify inject) #11725

Found by the package matrix re-run (#11723): fastify/inject throws … (reading 'once'). Reproduces 3/3 at 9e29f59 and on origin/main 9e05951.

On a CommonJS subclass of http.ServerResponse (built with http.ServerResponse.call(this, req) plus util.inherits), this.assignSocket(sock) does nothing, so this.connection stays undefined. The same code in a .ts file throws "not a ServerResponse" instead.

The 8-line repro is in RERUN-2026-09-30.md in #11723 (benchmarks/packages).