MtApi5: expose SymbolInfoMarginRate

[biohazardxxx]

Summary

Exposes the MQL5 built-in SymbolInfoMarginRate() through the MtApi5 connector. The initial and maintenance margin rates for a symbol (which depend on order type and direction) were not reachable from the .NET client; this adds the function end-to-end (C# client → EA handler → recompiled .ex5 → TestClient action).

New method on MtApi5Client:

Command id	Method
380	bool SymbolInfoMarginRate(string symbolName, ENUM_ORDER_TYPE orderType, out double initialMarginRate, out double maintenanceMarginRate)

Returns the native function's bool and writes the two rates to the out parameters.

Implementation notes

• Command id 380 — intentionally leaves a gap after GetSymbols = 306: dev does not yet contain the calendar ids 307–316 proposed in MtApi5: expose MT5 economic calendar (Calendar* functions) #301, and 380 stays clear of any collision with them.
• Mirrors the SymbolInfoSessionQuote (id 58) multi-value pattern. Wire format: request params {Symbol: string, OrderType: int}; response {RetVal: bool, Result: {Initial: double, Maintenance: double}}.
• C# client — deserializes via SendCommand<FuncResult<Dictionary<string, double>>> and reads Initial/Maintenance with TryGetValue; on a null or malformed response the out params are set to 0 and the method returns false (same fallback idiom as SessionQuote's DateTime.MinValue). ENUM_ORDER_TYPE already existed in Mt5Enums.cs and serializes as an int under the default Newtonsoft settings, so no new enums or DTOs were needed.
• MQL5 EA (mq5/MtApi5.mq5) — handler reads OrderType via GET_INT_JSON_VALUE, casts it to ENUM_ORDER_TYPE, initializes the two doubles to 0.0 before the call, and serializes the result via JSONNumber(double).
• TestClient — one button exercising SymbolInfoMarginRate on EURUSD / ORDER_TYPE_BUY, next to the existing SessionQuote/SessionTrade buttons.
• The diff is purely additive — no changes to existing behavior; files keep their existing CRLF line endings. A post-implementation review traced the command end-to-end (id match between Mt5CommandType.cs and the ADD_EXECUTOR registration, case-sensitive request-key match on both sides, response envelope exactly matching the FuncResult<Dictionary<string, double>> deserialization) and found no defects.

Testing

• MtApi5 and MtApi5TestClient build clean in Release (the TestClient's 332 nullable warnings are pre-existing on dev, none in the added code).
• MtApi5.mq5 compiles via MetaEditor with "Result: 0 errors"; the recompiled mq5/MtApi5.ex5 is committed.
• Not runtime-tested against a live MT5 terminal — verification was build/compile-level plus the code trace above; no live round-trip was run.

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