Create Dispute Counter

Start composing your evidence response to a dispute by creating a draft "counter". A counter holds your categorized evidence (each item is a canonical evidence category plus free text and/or attached files) for one round of a dispute, and is continuously saved as a draft so you don't lose work before submitting. There is **at most one open draft per dispute round**. Calling this when a draft already exists returns that draft rather than creating a duplicate, so it's safe to call on page load. Any attached `file_ids` must reference files you uploaded via the Files API; referencing a file you don't own returns `422`.