Separate request schemas for watchlist create and update

Trading API

Watchlist create and update requests now use separate schemas that reflect each operation's requirements:

• On create, name is required.
• On update, name is optional, but at least one of name or symbols must be provided.
• On update, symbols replaces the watchlist's existing assets with the new list.

Affected Endpoints

• POST /v2/watchlists
• PUT /v2/watchlists/{watchlist_id}
• PUT /v2/watchlists:by_name