=== GET /books/null-demo/jspecify-honest/0 (expect 200, no NPE -- compiler forced the null check) ===
HTTP/1.1 200 
Content-Type: application/json
Content-Length: 13
Date: Thu, 08 Oct 2026 05:30:33 GMT

{"length":-1}=== GET /books/null-demo/jspecify-honest/5 (non-null id, expect real length) ===
HTTP/1.1 200 
Content-Type: application/json
Content-Length: 12
Date: Thu, 08 Oct 2026 05:30:33 GMT

{"length":7}