Asset margin requirement fields documented as decimal strings

Broker API

margin_requirement_long and margin_requirement_short on the Asset schema are now documented as decimal strings, matching the other decimal-as-string fields.

• Both fields carry the decimal format.
• Descriptions state that the percentage is encoded as a decimal string.
• Examples added (100 for long, 30 for short).

The returned values are unchanged.

Affected Endpoints

• GET /v1/assets
• GET /v1/assets/{symbol_or_asset_id}
• GET /v1/trading/accounts/{account_id}/watchlists/{watchlist_id}
• POST /v1/trading/accounts/{account_id}/watchlists
• POST /v1/trading/accounts/{account_id}/watchlists/{watchlist_id}
• PUT /v1/trading/accounts/{account_id}/watchlists/{watchlist_id}