Fall back to LIST when XLIST advertises but returns unusable data

[monperrus]

Bug

JMAP::Sync::Common::folders() crashes with:

Can't use string ("ok") as an ARRAY ref while "strict refs" in use
  at JMAP/Sync/Common.pm line 488

on a live Dovecot (poste.io) deployment that advertises the XLIST
capability in its CAPABILITY response but, when actually sent an
XLIST command, replies with just the tagged completion status
instead of well-formed folder tuples.

folders() picks the list command based purely on capability
advertisement:

my $listcmd = $imap->capability()->{xlist} ? 'xlist' : 'list';
my @folders = $imap->$listcmd('', '*');

Reproduced directly against the live server with Mail::IMAPTalk:

$imap->list('', '*')   # → 6 well-formed folder tuples, as expected
$imap->xlist('', '*')  # → ('ok')  -- just the completion status

Since @folders then contains a single non-ref string 'ok', the
foreach loop's @{$folder->[0]} dereference dies with the error
above, aborting account setup entirely (POST /api/accounts returns
201 with a "setup failed: ..." warning and the account never
syncs any folders/messages).

Fix

After calling the capability-selected command, detect a malformed
XLIST result (any non-arrayref element) and retry with the standard
LIST command, which is universally reliable. This only changes
behavior on the specific failure case — servers with working XLIST
are unaffected.

Testing

Verified end-to-end against the live deployment (poste.io mail server,
Dovecot backend) via the Docker image
(ghcr.io/jmapio/jmap-proxy:latest):

• Before fix: POST /api/accounts → 201 with
  "warning":"setup failed: Can't use string (\"ok\") as an ARRAY ref...",
  account left with 0 folders / 0 messages.
• After fix: POST /api/accounts → clean 201, account synced
  6 folders / 49 messages successfully.

Also confirmed this is not a general "second IMAP command on the
connection" issue — namespace() and plain list() both work
reliably on this server; only the generic xlist dispatch returns
unusable data.