Refactor ISSQN calculation to use TributacaoIssqn class for consistency

[Copilot]

Addresses feedback on PR #42 to make ISSQN calculation in CalculaBaseCalculoIbsCbs consistent with how other taxes (PIS, COFINS, ICMS) are calculated.

Changes

• Replace manual ISSQN calculation with TributacaoIssqn class - Ensures discount handling respects _tipoDesconto parameter through proper abstraction
• Add error handling with try-catch - Returns 0 on calculation errors to prevent IBS/CBS base calculation failures
• Pass false to Calcula() - Skips retention calculations (ValorRetPis, ValorRetCofins, etc.) since only ISSQN value is needed for base deduction

Before:

private decimal CalcularValorIssqn()
{
    if (_tributavel.IsServico && _tributavel.PercentualIssqn > 0)
    {
        var baseCalculoIssqn = (_tributavel.ValorProduto * _tributavel.QuantidadeProduto) - _tributavel.Desconto;
        return baseCalculoIssqn * _tributavel.PercentualIssqn / 100;
    }
    return 0m;
}

After:

private decimal CalcularValorIssqn()
{
    if (_tributavel.IsServico && _tributavel.PercentualIssqn > 0)
    {
        try
        {
            var tributacaoIssqn = new TributacaoIssqn(_tributavel, _tipoDesconto);
            return tributacaoIssqn.Calcula(false).Valor;
        }
        catch
        {
            return 0m;
        }
    }
    return 0m;
}

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