Support implicit operators in method argument matching

[Copilot]

When a method parameter type differs from the passed argument type but an implicit operator exists between them, the parser failed with "No applicable method exists" — unlike plain C# which resolves such calls automatically.

Root cause

ExpressionPromoter.Promote (used by MethodFinder.IsApplicable to validate argument/parameter compatibility) had no knowledge of op_Implicit operators, so methods requiring implicit conversion were never considered applicable.

Changes

• ExpressionPromoter.cs — After TypeHelper.IsCompatibleWith fails, scan op_Implicit methods on both the source and target types. If a matching operator is found (returnType → type), return Expression.Convert(sourceExpression, type) to wire up the conversion.

• DynamicExpressionParserTests.cs — Added MyStructWithImplicitOperator (struct with string ↔ MyStruct implicit operators), MyMethodsWithImplicitOperatorSupport (a [DynamicLinqType] static class), and a test covering the scenario from the issue.

Example

[DynamicLinqType]
public static class MyMethods
{
    public static string UsesMyStruct(MyStruct myStruct) => myStruct.Value;
}

public readonly struct MyStruct
{
    public static implicit operator MyStruct(string value) => new(value);
    public static implicit operator string(MyStruct s) => s.Value;
    public string Value { get; }
}

// Previously threw: "No applicable method 'UsesMyStruct' exists in type 'MyMethods'"
var parser = new ExpressionParser([], "MyMethods.UsesMyStruct(\"Foo\")", [], ParsingConfig.Default);
var result = Expression.Lambda<Func<string>>(parser.Parse(typeof(string))).Compile()(); // "Foo"

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[Copilot]

Done in d0cc881. Added TryFindImplicitConversionOperator(Type sourceType, Type targetType, out MethodInfo? implicitOperator) to TypeHelper.cs, updated ExpressionPromoter.Promote and ExpressionParser.HasImplicitConversion to use it.