fix: prevent template dictionary memory leak in ds-atleastonce demo

[Copilot]

Code review identified a memory leak: the _templates dictionary grew unboundedly because it was never cleaned up when orders completed, negating the bounded inbox fix.

Changes

• Template cleanup: Added removal of payment/inventory templates in HandleAck when both acks received, matching cleanup of _acks, _attempts, and _correlationIds dictionaries
• Hardcoded strings → constants: Extracted PaymentsNodeName and InventoryNodeName constants to ensure cleanup logic stays synchronized with template creation

// Now cleans up all order-related state atomically
_acks.TryRemove(msg.OrderId, out _);
_attempts.TryRemove(msg.OrderId, out _);
_correlationIds.TryRemove(msg.OrderId, out _);
_templates.TryRemove(new LogicalMessageKey(MessageKind.PlaceOrder, msg.OrderId, PaymentsNodeName), out _);
_templates.TryRemove(new LogicalMessageKey(MessageKind.PlaceOrder, msg.OrderId, InventoryNodeName), out _);

Without this fix, long simulations would still experience unbounded growth through template accumulation despite the bounded inbox.

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[dexcompiler]

LGTM