MtApi5: pending order management (BuyLimit/SellLimit/BuyStop/SellStop, OrderModify, OrderDelete)

[biohazardxxx]

Summary

Adds pending order management to the MtApi5 connector. The existing CTrade wrappers (Buy/Sell) only cover market orders — there was no way to place, modify or delete pending orders from the .NET client. This adds the remaining CTrade order methods end-to-end (C# client → EA handler → recompiled .ex5):

• bool BuyLimit(out MqlTradeResult? result, double volume, double price, string? symbol = null, double sl = 0.0, double tp = 0.0, ENUM_ORDER_TYPE_TIME typeTime = ORDER_TIME_GTC, DateTime? expiration = null, string? comment = null)
• bool SellLimit(...), bool BuyStop(...), bool SellStop(...) — same signature as BuyLimit
• bool OrderModify(out MqlTradeResult? result, ulong ticket, double price, double sl, double tp, ENUM_ORDER_TYPE_TIME typeTime = ORDER_TIME_GTC, DateTime? expiration = null, double stoplimit = 0.0)
• bool OrderDelete(out MqlTradeResult? result, ulong ticket)

Implementation notes

• Command ids 320–325 (BuyLimit…OrderDelete), matching the ADD_EXECUTOR registrations in mq5/MtApi5.mq5. Ids 307–316 are left free for the calendar PR (MtApi5: expose MT5 economic calendar (Calendar* functions) #301), hence the intentional 317–319 gap.
• Implementation mirrors the existing Buy/Sell pattern exactly: same RetVal + Result response body (MqlTradeResultToJson), consumed by the same FuncResult<MqlTradeResult> contract on the C# side.
• The client sends TypeTime as int and Expiration as unix seconds via Mt5TimeConverter.ConvertToMtTime(DateTime?), which returns 0 for null — matching CTrade's expiration = 0 default. Symbol/Comment are omitted from the payload when null so the EA falls back to Symbol() / "", exactly like Buy/Sell.
• EA handlers are verbatim copies of the Execute_Buy/Execute_Sell shape (GET_*_JSON_VALUE macros, CTrade instance, result via trade.Result()), with casts (ENUM_ORDER_TYPE_TIME)type_time / (datetime)expiration; tickets are read with GET_ULONG_JSON_VALUE, and OrderModify passes stoplimit as the last CTrade::OrderModify argument per its signature.
• TestClient additions are kept minimal: a BuyLimit action (fetches the EURUSD tick via SymbolInfoTick and places a 0.1-lot limit 50 pips below bid) and an OrderDelete action that reuses the existing ticket textbox in the CTrade tab. The other methods have no dedicated buttons.
• A post-implementation review traced the commands end-to-end (id match between Mt5CommandType.cs and the ADD_EXECUTOR registrations, case-sensitive JSON key match on both sides, response envelope identical to the existing Buy/Sell contract, ENUM_ORDER_TYPE_TIME values 0–3 matching MQL5, correct CTrade signatures) and found no defects.
• Merge note: this branch is based on dev without the calendar changes, so the committed mq5/MtApi5.ex5 does not include calendar support. If MtApi5: expose MT5 economic calendar (Calendar* functions) #301 lands first, MtApi5.mq5 should be recompiled once after merging this branch.

Testing

• MtApi5 and MtApi5TestClient build clean (Release, 0 errors; the TestClient's nullable warnings are pre-existing).
• MtApi5.mq5 compiles via MetaEditor with "Result: 0 errors"; the freshly recompiled mq5/MtApi5.ex5 is committed.
• The feature has not been runtime-tested against a live MT5 terminal — verification is the green .NET builds plus the clean MQL compile and the code trace above.

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