Replace $arrayElemAt with $unwind in chat log export aggregation

[Copilot]

Addresses feedback from PR #5831 regarding MongoDB aggregation in exportChatLogs.ts. The original fix using $arrayElemAt was semantically unclear and required understanding of nested array unwrapping behavior.

Changes:

• Added explicit $unwind stages for chatData and chatItemResponsesData after $lookup operations
• Replaced all { $arrayElemAt: ['$chatData.field', 0] } expressions with direct field access: $chatData.field
• Used preserveNullAndEmptyArrays: true to maintain behavior for chats without items

Before:

{
  $addFields: {
    messageCount: { $ifNull: [{ $arrayElemAt: ['$chatData.messageCount', 0] }, 0] },
    userGoodFeedbackItems: {
      $filter: {
        input: { $ifNull: [{ $arrayElemAt: ['$chatData.chatitems', 0] }, []] },
        as: 'item',
        cond: { $ifNull: ['$$item.userGoodFeedback', false] }
      }
    }
  }
}

After:

{ $unwind: { path: '$chatData', preserveNullAndEmptyArrays: true } },
{ $unwind: { path: '$chatItemResponsesData', preserveNullAndEmptyArrays: true } },
{
  $addFields: {
    messageCount: { $ifNull: ['$chatData.messageCount', 0] },
    userGoodFeedbackItems: {
      $filter: {
        input: { $ifNull: ['$chatData.chatitems', []] },
        as: 'item',
        cond: { $ifNull: ['$$item.userGoodFeedback', false] }
      }
    }
  }
}

The unwinding makes the pipeline structure explicit and simplifies field access throughout the aggregation.

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