[Bug]: user_ldap: getDomainDNFromDN() includes `count` element from ldap_explode_dn(), producing invalid DN passed to ldap_read()

Bug description

When files:scan (or any code path that resolves a user's primary group via getSID()) is executed for an LDAP/AD user, a PHP warning is emitted:

Error during scan: ldap_read(): Search: Invalid DN syntax

The error is cosmetic (it doesn't break authentication or file access), but it registers as an error in occ files:scan output and pollutes logs.

Root cause

getDomainDNFromDN() in apps/user_ldap/lib/Access.php calls ldap_explode_dn($dn, 0), which returns an array that includes a 'count' key with an integer value (e.g. 4). The foreach loop does not skip non-string elements, so after finding the first dc= component, the integer 4 is appended to $domainParts:

// ldap_explode_dn("cn=John Doe,ou=staff,dc=example,dc=com", 0) returns:
// [0=>'cn=John Doe', 1=>'ou=staff', 2=>'dc=example', 3=>'dc=com', 'count'=>4]

foreach ($allParts as $part) {
    if (!$dcFound && str_starts_with($part, 'dc=')) {
        $dcFound = true;
    }
    if ($dcFound) {
        $domainParts[] = $part;  // also appends integer 4 from 'count' key
    }
}
return implode(',', $domainParts);
// returns "dc=example,dc=com,4"  ← invalid DN

readAttribute("dc=example,dc=com,4", "objectsid") is then called → ldap_read($link, "dc=example,dc=com,4", ...) → "Invalid DN syntax".

Call stack (from occ files:scan -vvv)

ldap_read()
OCA\User_LDAP\LDAP->invokeLDAPMethod()
OCA\User_LDAP\Access->invokeLDAPMethod()
OCA\User_LDAP\Access->executeRead()              Access.php:245
OCA\User_LDAP\Access->readAttribute()            Access.php:196
OCA\User_LDAP\Access->getSID()                   Access.php:1840
OCA\User_LDAP\Group_LDAP->primaryGroupID2Name()  Group_LDAP.php:497
OCA\User_LDAP\Group_LDAP->getUserPrimaryGroup()  Group_LDAP.php:618
OCA\User_LDAP\Group_LDAP->getUserGroups()        Group_LDAP.php:689

Steps to reproduce

1. Configure Nextcloud with Active Directory (any AD domain)
2. Ensure at least one LDAP user exists and has logged in at least once
3. Run sudo -u www-data php occ files:scan --all -vvv
4. Observe: Error during scan: ldap_read(): Search: Invalid DN syntax

Expected behavior

getDomainDNFromDN() returns a valid DN (dc=example,dc=com) and no error is emitted.

Proposed fix

Skip non-string elements in the loop (one line change in apps/user_ldap/lib/Access.php):

foreach ($allParts as $part) {
    if (!is_string($part)) continue; // skip 'count' integer from ldap_explode_dn
    if (!$dcFound && str_starts_with($part, 'dc=')) {
        $dcFound = true;
    }
    if ($dcFound) {
        $domainParts[] = $part;
    }
}

Environment

• Nextcloud version: 34.0.0 (also reproduced against current master)
• PHP version: 8.5
• LDAP backend: Active Directory
• OS: Debian 13 (trixie)