Remove plural form heuristics from XmlDeserializer and use generic test names

[Copilot]

Removes experimental plural form matching logic from XmlDeserializer.HandleListDerivative() while preserving the core nested element fix. Refactors test classes from domain-specific names (Category/Categories) to generic names (Item/Items).

Changes

XmlDeserializer.cs

• Removed plural form attempts (name + "s", lowercase + "s") from container lookup
• Container matching now only uses property name directly
• Preserved Elements() vs Descendants() logic that fixes nested element inclusion bug
• Added IsValidXmlElementName() helper for generic type name validation
• Fixed RemoveNamespace() to filter null attributes
• Fixed RootElement selection to prefer shallowest match

Test classes (NestedElementTestClasses.cs)

• Category → Item
• CategoryContainer → ItemContainer
• CategoryWithAssociations → ItemWithGroup
• CategoryAssociations → ItemGroup
• CategoriesResponse → ItemsResponse
• All XML test data uses <item>, <items>, <subitems> instead of category terminology

Example

Before (with plural logic):

// Tried multiple plural variations
container = GetElementByName(root, (name.ToLower() + "s").AsNamespaced(Namespace));
container = GetElementByName(root, (name + "s").AsNamespaced(Namespace));

After (property name only):

// Direct property name lookup only
if (IsValidXmlElementName(propName)) {
    container = GetElementByName(root, propName.AsNamespaced(Namespace));
}

Tests validate proper handling of nested structures without relying on pluralization heuristics:

<items>
    <item>
        <id>1</id>
        <subitems>
            <item><id>2</id></item>  <!-- Not included in parent list -->
        </subitems>
    </item>
</items>

Original prompt

in branch copilot/fix-xml-deserializer-bugs remove in xmldeserializer tries with plural forms and in new test added make it more general samples avoid use category or associations but something like item items group subgroupt etc

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