Add a comment to an alert.

Comment Limits:

• Maximum length: 500 characters
• Maximum mentions: 50 users per comment

@Mentions: Comments support mentioning users using two formats:

• @{guid} - Mention by user ID (e.g., @a1b2c3d4-e5f6-7890-abcd-ef1234567890)
• @{email} - Mention by email address (e.g., @[email protected])

Email mentions are automatically converted to GUID format when the user is found. Invalid mentions (non-existent users or users without shipment access) are preserved in the comment text but excluded from notifications. All invalid mentions are returned in the response warnings.invalidMentions array. Valid mentioned users receive email and in-app notifications.