Forgejo answers 422 for six different causes on admin user create
(ErrUserAlreadyExist, ErrEmailAlreadyUsed, ErrNameReserved,
ErrNameCharsNotAllowed, ErrEmailInvalid, ErrNamePatternNotAllowed) and
several on repo create, but is_already_exists() treated every one of
them as a conflict. A reserved or otherwise-refused name silently
folded to Done, so CreateForgeUser reported success with no user
created, and the graph's real failure only surfaced one node later as
a misleading AddRepoMember error.
ensure_agent_user and ensure_org_repo now trust a 409 unconditionally
(folds_into_success) but confirm a 422 with a follow-up user_get /
repo_get before folding it to success; an unconfirmed 422 fails with
forgejo's own message at error level. Webhook registration still uses
the old is_already_exists — it has no comparable follow-up read, so it
is out of scope here.
Closes#4678
295925abb6