feature: add TerminalLink IsCfdTrade order property

[AlexCatarino]

Description

Adds IsCfdTrade to TerminalLinkOrderProperties so an algorithm can book an EMSX order as a contract for differences instead of a regular trade.

On the EMSX trading ticket this is the Booking Type drop-down, whose options are Regular, CFD and TRS:

• Only the CFD booking has an element in the EMSX API — EMSX_CFD_FLAG, a STRING ORDER-level field taking "0"/"1".
• There is no booking-type element, and nothing for TRS (total return swap), anywhere in Bloomberg's EMSX element reference or in its CreateOrderAndRouteEx samples. EMSX_ASSET_CLASS is likewise limited to EQTY/OPT/FUT/MULTILEG_OPT.

Hence a bool rather than a three-valued enum. It defaults to false, which is the EMSX default and the value for which the brokerage sends no flag at all, so existing orders are unchanged on the wire. The XML summary is Bloomberg's own wording for the element.

AdditionalProperties

IsCfdTrade is a pass through over a new AdditionalProperties dictionary of EMSX element name → value, following BloombergFixOrderProperties, so the typed property and the raw element are one store and cannot disagree. The brokerage PR drains the dictionary onto the request, which also lets an algorithm set any EMSX element without waiting on a Lean release.

Two deliberate differences from FixOrderProperties:

• No setter. A plain Python dict is not convertible to Dictionary<string, string> — pythonnet raises "'dict' value cannot be converted" — and the only way to assign wholesale would be Dictionary[str, str]() from Python, which is not an idiom we want to document. Swapping the instance would also detach the typed properties from their store. The dictionary starts empty and is add/remove only; clear() resets it, and works from both C# and Python (covered by a test).
• Clone() is overridden to deep copy it. OrderProperties.Clone() is a MemberwiseClone, which would share the dictionary — and BrokerageExtensions.RemoveLocateFromNonShortOrder clones precisely so its edits never reach the caller's instance. There is a regression test.

Possible follow-up: ExtendedDictionary<TKey, TValue> would give the dictionary proper Python semantics; it is abstract, so it would need a concrete <string, string> subclass. Out of scope here, raising it as an option.

Related Issue

QuantConnect/Lean.Brokerages.TerminalLink#135, companion PR QuantConnect/Lean.Brokerages.TerminalLink#136.

Requires Documentation Change

Yes — IsCfdTrade and AdditionalProperties should be listed with the other TerminalLink order properties.

Tests

TerminalLinkOrderPropertiesTests, 10 passing, covering the default, the Clone isolation, and the Python surface: is_cfd_trade, writing an entry through additional_properties, clear(), and that the dictionary cannot be replaced.

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