Fix XmlDeserializer to correctly handle nested XML elements with duplicate tag names

[qodo-free-for-open-source-projects]

Description

• XmlDeserializer incorrectly flattens nested lists by using Descendants() which includes elements at all depths, causing deeply nested items to be included in parent collections
• RootElement selection fails when XML contains duplicate element names at different nesting levels, throwing InvalidOperationException or selecting wrong element
• RemoveNamespace() processing includes null values causing potential issues, and unnecessary null-forgiving operators reduce code safety

Deliverables

• Fix list handling: Modify HandleListDerivative to detect container elements and use Elements() for direct children only, with Descendants() as fallback for backward compatibility
• Fix RootElement: Implement shallowest-match preference by trying Element() first, then DescendantsAndSelf() ordered by ancestor count
• Improve robustness: Filter null values in RemoveNamespace, remove null-forgiving operators, add validation for generic type names in XML element names
• Add test coverage: Create comprehensive test suite covering nested lists, duplicate element names, and complex nesting scenarios