Hex float "X" format doesn't round-trip in built-in cultures with directional marks in their sign strings #135039

Description

In .NET 11, double/float/Half formatted with the "X"/"x" format cannot be parsed back with NumberStyles.HexFloat using the same culture when the culture's PositiveSign/NegativeSign is not plain "+"/"-" (for example he-IL, ar-*, fa-IR, ur-PK). The formatter always writes the exponent sign as ASCII +/-, but the parser only accepts the culture's PositiveSign/NegativeSign there.

Reproduction Steps

using System;
using System.Globalization;

foreach (string name in new[] { "en-US", "ar-AE" })
{
    var culture = new CultureInfo(name);
    foreach (double value in new[] { Math.PI, -0.1 })
    {
        string s = value.ToString("X", culture);
        bool ok = double.TryParse(s, NumberStyles.HexFloat, culture, out _);
        Console.WriteLine($"{name}: {s} -> parses back: {ok}");
    }
}

Expected behavior

Both cultures print parses back: True, as CultureInfo.InvariantCulture does.

Actual behavior

en-US: 0X1.921FB54442D18P+1 -> parses back: True
en-US: -0X1.999999999999AP-4 -> parses back: True
ar-AE: 0X1?921FB54442D18P+1 -> parses back: False
ar-AE: ?-0X1?999999999999AP-4 -> parses back: False

double.Parse with ar-AE throws FormatException: The input string '0X1.921FB54442D18P+1' was not in a correct format.

Regression?

No, hex floating-point formatting and parsing are new in .NET 11.

Known Workarounds

Use CultureInfo.InvariantCulture for both formatting and parsing.

Configuration

.NET 11.0.0-rc.1.26425.128, linux-x64

Other information

• This came up briefly in the Add support for floating-point hex formatting and parsing #124139 review (thread) in the context of custom NumberFormatInfo instances; it also affects built-in cultures, since several CLDR cultures prefix their sign strings with directional marks (e.g. ar-AE PositiveSign is "\u200E+").