$ curl -i http://localhost:8080/api/products/42 -H 'If-None-Match: "4e47fa7e"'

HTTP status: 304
Body: '' (empty)

# WebRequest.checkNotModified(...) wrote the 304 and short-circuited the handler BEFORE
# the controller method's own body ran any further -- this is the "deep cache" case: a
# real database read behind productService.findById(id) is only avoided if you compute
# the comparison value (e.g. a stored version/timestamp) more cheaply than the full fetch,
# which this in-memory demo simplifies but a real service must design around explicitly.
