### Issue:
- In the Attendance Gantt view, users can see all employees in the company, including those they are not allowed to manage or view.
- While access rules prevent users from seeing attendance records for these employees, the employees' names still appear in the Gantt view as group headers.
- This creates a misleading user experience and potentially exposes restricted information (employee names).

### Steps To Reproduce:
1. Log in as Marc Demo.
2. Open the attendance App.
3. Observe that all employees are shown as group headers.

### Solution:
- Since Odoo 18.0, the `employee_id` field in `hr.attendance` was given a `group_expand='_read_group_employee_id'`. this method returns all employees in the allowed companies, without checking if the user actually manages them.
- We updated the `group_expand` method `_read_group_employee_id` to restrict the list of employees shown in the Gantt view.
Now, users will only see employees for whom they are set as the `attendance_manager_id`, unless they belong to the HR Officer group or are superusers, in which case they continue to see all employees.
- This ensures that group headers in the Gantt view match the user's actual access rights.

opw-4570867

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