fix(LiveChatTextMessage): Make some fields optional

[jonz94]

• Mark menu_endpoint property as optional
• Mark context_menu_accessibility_label property as optional

Example RawNode data:

{
  "liveChatTextMessageRenderer": {
    "message": {
      "runs": [
        {
          "text": "!"
        }
      ]
    },
    "authorName": {
      "simpleText": "庫洛姆•Kuromu"
    },
    "authorPhoto": {
      "thumbnails": [
        {
          "url": "https://yt4.ggpht.com/vKALKrLFo09MqpQASaS1hvOBVN9FRMn4aUPxf8UU8OzQCMXT8-C-XilHREtyV-wCeGwFee8_0A=s32-c-k-c0x00ffffff-no-rj",
          "width": 32,
          "height": 32
        },
        {
          "url": "https://yt4.ggpht.com/vKALKrLFo09MqpQASaS1hvOBVN9FRMn4aUPxf8UU8OzQCMXT8-C-XilHREtyV-wCeGwFee8_0A=s64-c-k-c0x00ffffff-no-rj",
          "width": 64,
          "height": 64
        }
      ]
    },
    "id": "ChwKGkNJX0M4dHFSOVlvREZUelN3Z1FkRjlVaWZB",
    "timestampUsec": "1736855168796805",
    "authorBadges": [
      {
        "liveChatAuthorBadgeRenderer": {
          "icon": {
            "iconType": "OWNER"
          },
          "tooltip": "Owner",
          "accessibility": {
            "accessibilityData": {
              "label": "Owner"
            }
          }
        }
      }
    ],
    "authorExternalChannelId": "UC2ZWggon1NOT2TGaVUMzY7A",
    "trackingParams": "CAYQl98BIhMIsP-Rz6aBiwMVe2r1BR3LvzM3"
  }
}