Fix Sse Memory Leak

Fix memory leak where every aborted SSE connection pinned the full request/response graph on Node 20, caused by AbortSignal.any() in sse.ts retaining its source signals indefinitely (see nodejs/node#54614, nodejs/node#55351). Also clear the setTimeout(abort) timer in entry.server.tsx so successful HTML renders don't pin the React tree for 30s per request.