fix(events): serialize nested entities in webhook payloads

[chiliec]

• Resolves: [bug] Webhook events do not serialize labels in mongo filters #8245
• Target version: main

Summary

Webhook event filters like {"event.card.labels.title": "foo"} crash in webhook_listeners with:

PHPMongoQuery::_executeOperatorOnElement(): Argument #4 ($document) must be of type array, OCA\Deck\Db\Label given

ACardEvent::getWebhookSerializable() returned $this->card->jsonSerialize(), which is only one level deep: labels, assignedUsers, attachments etc. stay as Label/Assignment objects inside the array. json_encode would resolve them later, but PHPMongoQuery runs the filter on the raw array before encoding and can only traverse arrays.

Fix: serialize the card / acl through json_encode+json_decode so the payload handed to the webhook listener is plain nested arrays. Same change on AAclEvent for consistency (its participant relation has the same shape).

Validation

Reproduced against a local nextcloud/server master checkout with webhook_listeners' own PHPMongoQuery:

• before: executeQuery(['event.card.labels.title' => 'foo'], $data) → the exact TypeError from the issue
• after: returns true for a matching label title, false for a non-matching one

Regression test testCardEventSerializesNestedEntitiesAsArrays added to the existing WebhookCompatibleEventsTest; it fails on main (Failed asserting that OCA\Deck\Db\Label Object ... is of type array) and passes with this change.

• composer run test:unit — 425 tests, 4088 assertions, OK (the single SUBJECT_LABEL_DELETE warning is pre-existing on main)
• composer run cs:check — clean
• composer run psalm — no errors

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Documentation (manuals or wiki) has been updated or is not required

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