DeleteBucket answers from a raw per-disk residue scan rather than from a
listing, so it can refuse for a reason no S3 request can observe: the
client drains every version the API will show, DeleteBucket still returns
BucketNotEmpty, and the client-visible message is the generic "The bucket
you tried to delete is not empty" for every blocker kind.
The server does know which residue blocked it, and where — that is what
`bucket_delete_blocked` carries. But it was emitted at `debug`, below
both the `error` DEFAULT_LOG_LEVEL and the `info` the CI s3-tests lane
runs at, so it was never actually written down. An intermittent
BucketNotEmpty in that lane leaves a server log with no trace of the
refusal at all, which is not a diagnosable state: confirmed against the
artifact log of a failing run, where the rejected bucket appears only in
span-close lines and the blocker event is absent entirely.
Split the blocker kinds by whether the client can still reach the
residue. A visible version or a tier free-version is an ordinary 409 —
the bucket really is not empty and the caller can list and delete what is
left — so that stays at `warn`. UnknownXlMeta, OrphanDirectory, and
DiagnosticBudgetExceeded are on-disk state no S3 request can remove; that
is a server-side integrity problem and is now reported at `error`, with
the blocker kind, the residue counts, and the sample path.
This does not change what DeleteBucket accepts or rejects, and does not
retry or suppress anything — it makes the existing diagnosis reachable.
Refs #7005, #7010
