Event formatters should return date/datetime with timezone

Problem/Motivation

REST API endpoint "/bat_api/rest/calendar-events" returns date/datetime without timezone information.

[
    {
        "resourceId": 3,
        "bat_id": 44,
        "start": "2025-11-14T00:00:00",
        "end": "2025-11-14T00:59:00",
        "title": "N\/A",
        "color": "#f04b4b",
        "blocking": 1,
        "fixed": 1,
        "editable": false,
        "type": "availability_hourly"
    },
    ...
]

Since good practise is to operate on UTC it should return UTC timezone.

Steps to reproduce

Proposed resolution

Modify Event formatters files:
- /web/modules/contrib/bat/modules/bat_fullcalendar/src/FullCalendarFixedStateEventFormatter.php
- /web/modules/contrib/bat/modules/bat_fullcalendar/src/FullCalendarOpenStateEventFormatter.php

They should return date/datetime with timezone information, since default in the system is UTC, return should look like this:

[
    {
        "resourceId": 3,
        "bat_id": 44,
        "start": "2025-11-14T00:00:00Z",
        "end": "2025-11-14T00:59:00Z",
        "title": "N\/A",
        "color": "#f04b4b",
        "blocking": 1,
        "fixed": 1,
        "editable": false,
        "type": "availability_hourly"
    },
    ...
]

or

[
    {
        "resourceId": 3,
        "bat_id": 44,
        "start": "2025-11-14T00:00:00+00:00",
        "end": "2025-11-14T00:59:00+00:00",
        "title": "N\/A",
        "color": "#f04b4b",
        "blocking": 1,
        "fixed": 1,
        "editable": false,
        "type": "availability_hourly"
    },
    ...
]

Remaining tasks

User interface changes

API changes

Data model changes

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