CVE-2026-7768: @fastify/accepts-serializer Vulnerable to Denial of Service via Unbounded Accept Header Cache Growth

@fastify/accepts-serializer cached serializer-selection results keyed by the request Accept header without a size limit or eviction policy. A remote unauthenticated client could send many distinct but matching Accept header variants to make the cache grow unbounded. Under sustained load, this can exhaust the Node.js heap and crash the process.