fix(services/dbfs): make listed entry paths relative to the root

[PDGGK]

Which issue does this PR close?

None. This is the second half of #8079, kept separate because it changes what users see rather than what goes on the wire.

Rationale for this change

The lister emits status.path straight from the response:

for status in decoded_response.files {
    let entry: oio::Entry = match status.is_dir {
        true => {
            let normalized_path = format!("{}/", status.path);
            ...
            oio::Entry::new(&normalized_path, meta)
        }
        false => {
            ...
            oio::Entry::new(&status.path, meta)
        }
    };

DbfsCore::list sends the prefix as build_rooted_abs_path(&self.root, path), and DBFS echoes absolute paths under that prefix. So every entry comes back carrying the root and a leading slash — /data/dir/f.txt where the entry contract asks for dir/f.txt. Even at the default root the leading slash is wrong.

alluxio is the exact analogue in this tree — same API shape, appends / for directories, then relativises:

let path = if file_info.folder { format!("{path}/") } else { path };
ctx.entries.push_back(Entry::new(
    &build_rel_path(&self.core.root, &path),
    file_info.try_into()?,
));

Every other HTTP service lister does the same; dbfs is the only one that omits it. Nothing downstream repairs it either — oio::Entry::with only rewrites the empty string to "/".

What changes are included in this PR?

build_rel_path applied once per entry, hoisted above the match because both arms need it. Three lines.

Tests

None — the change makes this lister do what alluxio and the rest already do, and there is no directory under .github/services for dbfs, so nothing exercises the listing path either way.

cargo build, cargo clippy -p opendal-service-dbfs --all-targets (zero warnings) and cargo fmt --all -- --check are clean.

Are there any user-facing changes?

Yes: listed entries are relative to the operator root, as the entry contract requires, instead of carrying the root and a leading slash.