Asset margin requirement fields documented as decimal strings

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 format: decimal.
• Examples added (100 for long, 30 for short).
• Descriptions state the values are encoded as decimal strings. Returned values are unchanged.

Broker API

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

Trading API

• GET /v2/assets
• GET /v2/assets/{symbol_or_asset_id}
• POST /v2/watchlists
• GET /v2/watchlists/{watchlist_id}
• POST /v2/watchlists/{watchlist_id}
• PUT /v2/watchlists/{watchlist_id}
• DELETE /v2/watchlists/{watchlist_id}/{symbol}
• GET /v2/watchlists:by_name
• POST /v2/watchlists:by_name
• PUT /v2/watchlists:by_name