fix: gofmt s3storage_test.go

Remove trailing blank line from core/pkg/storage/s3storage_test.go so
that 'gofmt -l' (and 'just fmt-check') produces no diff.

This unblocks the 'format-check' CI job for PR #3706.

Signed-off-by: Cursor Agent <cursor@opencost.io>

Co-authored-by: Alex Meijer <ameijer@users.noreply.github.com>

core/pkg/storage/s3storage_test.go

@@ -96,4 +96,3 @@ func TestSetGetObjectRange(t *testing.T) {
 		})
 	}
 }
-