Fix InvalidCastException in Then<U>() for non-IConvertible types

[Copilot]

Then<U>() throws InvalidCastException when U doesn't implement IConvertible (e.g., custom record types). This broke in v1.4.0 when replacing Discard<U>().

Changes

• Type conversion strategy: Check if U implements IConvertible at parser construction time
  • If yes: attempt Convert.ChangeType() with fallback to default(U) on failure
  • If no: directly return default(U) using the value constructor (avoids lambda overhead)
• Performance: Move reflection check from parse-time to construction-time

Example

// Now works for custom types
public abstract record BaseExp;
var parser = commentParser.Then<BaseExp>();  // Previously threw InvalidCastException

// Still works for convertible types  
var intToDecimal = Literals.Integer().Then<decimal>();  // Converts int to decimal

Maintains backward compatibility with existing type conversions while enabling non-convertible types.

Original prompt

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This section details on the original issue you should resolve

<issue_title>InvalidCastException when using .Then() where T is not IConvertible</issue_title>
<issue_description>Hi, after we updated to version 1.4.0 of parlot we started to get System.InvalidCastException : Object must implement IConvertible. exceptions in the code. It looks like it happens when using .Then<T>() where T is not IConvertible. Was this intentional, that Then now requires a IConvertible to discard the result?

Here is a snippet of test code which now fails after 1.4.0. The first case with the // A comment fails, but the other one is ok.

using Parlot.Fluent;
using static Parlot.Fluent.Parsers;

public class TestClass
{
    [Theory]
    [InlineData("""
        // A Comment
        var a = 'test'
        """)]
    [InlineData("""
        var a = 'test'
        """)]
    public void SimpleTest(string testString)
    {
        var commentParser = Terms.Text("//").And(AnyCharBefore(OneOf(Literals.Char('\n'), Literals.Char('\r'))));
        var varKeyword = Terms.Text("var");
        var identifier = Terms.Identifier();
        var equal = Terms.Char('=');
        var str = Terms.String();

        var expressionParser = varKeyword.And(identifier).And(equal).And(str).Then<BaseExp?>(a => new AssignExpression(a.Item2.ToString()!, a.Item4.ToString()!));

        var simpleParser = OneOf(commentParser.Then<BaseExp>(), expressionParser);

        var result = simpleParser.Parse(testString);
    }

    public abstract record BaseExp;

    public record AssignExpression(string Name, string Value) : BaseExp;
}

We were able to work around it by calling .Then(default(BaseExp)) instead, for ignoring the value</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes InvalidCastException when using .Then<T>() where T is not IConvertible #235

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