fix(fastify-sse): add X-Accel-Buffering header to per-request startSSE

Aligns with hub.ts which already includes this header. Prevents
nginx/Traefik from buffering SSE chunks behind a reverse proxy.

packages/fastify-sse/src/per-request.test.ts

@@ -27,6 +27,7 @@ describe('per-request SSE helpers', () => {
         'Content-Type': 'text/event-stream',
         'Cache-Control': 'no-cache',
         Connection: 'keep-alive',
+        'X-Accel-Buffering': 'no',
       });
     });
 

packages/fastify-sse/src/per-request.ts

@@ -11,6 +11,7 @@ export function startSSE(reply: FastifyReply): void {
     'Content-Type': 'text/event-stream',
     'Cache-Control': 'no-cache',
     Connection: 'keep-alive',
+    'X-Accel-Buffering': 'no',
   });
   reply.hijack();
 }