[Backport 5.x] fix: callNotFound should run not-found preHandler regardless of registration order - #6973

…tration order (#6965)

setContext() in four-oh-four.js takes a detached shallow snapshot of the
selected not-found context. When a route that calls reply.callNotFound()
is registered before setNotFoundHandler(), the snapshot is taken before
the not-found handler's preReady callback populates its lifecycle hooks,
so the copied context retains preHandler: null. reply.callNotFound()
then
dispatches the not-found handler directly, skipping the preHandler that
setNotFoundHandler() declares.

This change uses the selected context as the prototype of the
route-specific
404 context instead of copying it, keeping the route's own onSend value,
so
the route-specific context stays linked to the live one and later hook
population is visible.

Adds a regression test covering the route-first declaration order.

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Co-authored-by: mcollina <[email protected]>
(cherry picked from commit 4176096)