Update a to-do

PATCH /v1/activity-todos/:id accepts a non-empty body with any of deadline, title, description, responsibleId, parentActivityId, or pingOffsets. The CRM owner is read from the existing activity. Other providers return 404.

A sole deadline, description, or responsibleId uses the matching Bitrix24 point method. Other combinations use crm.activity.todo.update: the service reads the activity and supplies its existing title, description, and deadline when omitted from PATCH. This prevents silent field erasure. The service also preserves existing reminders, files, and activity blocks. Set color when creating a to-do: PATCH with a valid colorId returns 422 because full update could erase settings added concurrently. A color outside the palette returns 400. A completed to-do returns 422.

Success: HTTP 200, { "success": true, "data": <updated activity> }.