[bug] Webhook events do not serialize `labels` in mongo filters

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Describe the bug
When a webhook is registered and filters on a dot-notation query for a label attribute, the query fails because the labels are not serialized properly.

To Reproduce

1. Register the following webhook:

{
    "httpMethod": "POST",
    "uri": "https://<INSERT TRUSTED WEBHOOK LOCATION>",
    "event": "OCA\\Deck\\Event\\CardUpdatedEvent",
    "eventFilter": {
        "event.card.labels.title": "foo"
    },
    "authMethod": null,
    "authData": null,
    "tokenNeeded": null
}

2. Create a card and assign it a label.

3. Monitor logs for the following error:

OCA\WebhookListeners\Service\PHPMongoQuery::_executeOperatorOnElement(): Argument #4 ($document) must be of type array, OCA\Deck\Db\Label given, called in /config/www/nextcloud/apps/webhook_listeners/lib/Service/PHPMongoQuery.php on line 220 in file '/config/www/nextcloud/apps/webhook_listeners/lib/Service/PHPMongoQuery.php' line 198

Expected behavior
The webhook should be able to filter on the label attributes.

Screenshots
N/A

Client details:

• OS: macOS 26
• Browser: Firefox
• Version: 153.0.3
• Device: macbook pro m2

Login as admin user into your Nextcloud and access
http://example.com/index.php/settings/integrity/failed
paste the results here.

If you have access to your command line run e.g.:
sudo -u www-data php occ app:list
from within your Nextcloud installation folder

If you have access to your command line run e.g.:
sudo -u www-data php occ config:list system
from within your Nextcloud installation folder

or

Insert your config.php content here
Make sure to remove all sensitive content such as passwords. (e.g. database password, passwordsalt, secret, smtp password, …)

Insert your Nextcloud log here

Insert your browser log here, this could for example include:

a) The javascript console log
b) The network log
c) ...

Start in apps/webhook_listeners/lib/Service/PHPMongoQuery.php, especially the call around line 220 and _executeOperatorOnElement() around line 198. Reproduce the Deck CardUpdatedEvent webhook with a dot-notation filter on event.card.labels.title, then verify that label attributes serialize correctly and filtering no longer raises the reported type error.