Handle unknown liquidity as None (#109)

TWS will sometimes return UNSET_INTEGER (2**31 -1). If we panic, it will prevent enumerating the orders. This is compliant with the C# client behavior: https://github.com/quantrocket-llc/ibapi/blob/e3542aa3e05a2b752eb33742f81a50d4ac0aa1d8/source/csharpclient/client/Execution.cs#L31
Also, we note that the C# comment lists 0 = Unknown.

src/orders.rs

@@ -864,11 +864,10 @@ pub enum Liquidity {
 impl From<i32> for Liquidity {
     fn from(val: i32) -> Self {
         match val {
-            0 => Liquidity::None,
             1 => Liquidity::AddedLiquidity,
             2 => Liquidity::RemovedLiquidity,
             3 => Liquidity::LiquidityRoutedOut,
-            _ => panic!("unsupported Liquidity({val})"),
+            _ => Liquidity::None,
         }
     }
 }