fix(orders): preserve unknown execution liquidity codes

[faysou]

Summary

Map the documented execution liquidity codes as before, including wire code zero as Liquidity::None. Preserve every other value as Liquidity::Unknown(i32) so callers can log, store, or reject an unsupported code without silently treating it as missing data.

flowchart LR
    Wire[Execution liquidity code] --> Known{Code 0 to 3?}
    Known -- Yes --> Variant[Known Liquidity variant]
    Known -- No --> Unknown[Liquidity::Unknown with original code]

Loading

The conversion remains infallible while retaining the wire value for forward compatibility.

Testing

• cargo test -p ibapi --features sync.
• Unit coverage for zero, positive unknown, and negative unknown codes.

[wboayue]

Thanks @faysou! Nice catch on preserving the wire value — verified against the official clients that 0–3 is the complete documented set, so Unknown(i32) is exactly the right forward-compat move. I pushed a changelog entry to the branch; will land this with the next breaking release since Liquidity is deliberately exhaustive.