Fix Issue 886: VB -> C# wrong conversion of optional struct or decimal ref parameter

[GrahamTheCoder]

This PR fixes an issue where converting Optional ByRef VB.NET parameters initialized with Nothing for custom structures or numeric Decimal defaults emitted C# code that fails to compile due to C# compiler limitations with [DefaultParameterValue].

Changes:

1. ExpressionNodeVisitor.cs: For [DefaultParameterValue], decimal parameter primitive defaults now properly emit an equivalent constant int or double valid attribute expression, without the decimal m suffix which is disallowed. For generic structs (TypeKind.Struct and SpecialType.None), no DefaultParameterValue attribute will be generated as custom structs with non-null defaults fail natively on C#.
2. ArgumentConverter.cs: Whenever default parameters expand for variable definitions around callers (ref arguments), non-nullable structs initialized to null instead will yield their expected default initialization.
3. Added the expected unit test OptionalByRefParameterAsync886 and verified it passes correctly alongside all project suites.

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PR created automatically by Jules for task 15338263271799288852 started by @GrahamTheCoder

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