4 cases: zero/negative id -> BadRequest; unknown id -> null
(controller maps to 404); existing id returns the Post.
GetPost takes int id directly (no Request object - the value is
trivial and controllers pull it from a route param).
5 cases: zero/negative userId -> BadRequest; user with no posts
-> []; returns only requested user's posts (filters out other
authors); ordered newest-first by createdAt. fails red.
7 cases: null + whitespace title -> BadRequest; null +
whitespace body -> BadRequest; valid request returns Post with
correct userId/title/body and createdAt = clock.now(); the post
is findable via the repo afterwards; title and body get trimmed
of leading/trailing whitespace. fails red - CreatePost class
absent.
