TastytradeBrokerageModel and TradeStationBrokerageModel declared
NotSupportedCrossZeroOrderTypes as an expression-bodied property, allocating a
new constant HashSet on every CanSubmitOrder call (used per order in
BrokerageExtensions.ValidateCrossZeroOrder). Make it a static readonly field,
built once, matching the existing BrokerageExtensions.DefaultNotSupportedCrossZeroOrderTypes
pattern. Set contents are unchanged, so cross-zero validation behavior is identical.
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>