Fix XML serialization of xs:list elements to respect white-space separation

[Copilot]

When serializing string[] properties with [XmlText] attribute, array elements were being concatenated without spaces, and deserialization was treating the entire content as a single element instead of splitting on whitespace as required by XML Schema list datatype specifications.

Problem

public class TestClass
{
    [XmlText]
    public string[] Items { get; set; }
}

var original = new TestClass { Items = new[] { "a", "b", "c", "d" } };
// Serialization produced: "abcd" (concatenated, no spaces)
// Deserialization produced: ["abcd"] (single element)

Expected Behavior

According to XML Schema list datatypes, elements should be space-separated:

// Serialization should produce: "a b c d" (space-separated)
// Deserialization should produce: ["a", "b", "c", "d"] (individual elements)

Solution

Serialization fix: Modified ReflectionXmlSerializationWriter.WriteText() to detect string[] arrays and join them with spaces using string.Join(" ", stringArray) before writing the text content.

Deserialization fix: Modified ReflectionXmlSerializationReader.WriteMemberText() to detect string[] arrays and split space-separated text using textValue.Split((char[]?)null, StringSplitOptions.RemoveEmptyEntries).

Test update: Updated the existing XML_TypeWithXmlTextAttributeOnArray() test to expect the correct behavior where ["val1", "val2"] serializes as "val1 val2" and deserializes back to ["val1", "val2"].

The changes are minimal and surgical, affecting only the specific case of string[] properties with [XmlText] attribute, ensuring existing functionality remains unchanged.

Fixes #115837.

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