Fix XML object serializer missing itemsName wrapping for arrays

[joheredi]

Problem

The buildXmlObjectModelSerializer (used for nested models) was generating flat mappings for array properties
without wrapping items under their itemsName element. This caused incorrect XML where array items were placed
directly under the wrapper element, missing the intermediate item element.

For example, given:

model ArrowConfiguration {
  @Xml.name("Schema") schema: ArrowField[];
}

@Xml.name("Field")
model ArrowField { ... }

Before (incorrect): { "Schema": items.map(...) } After (correct): { "Schema": { "Field": items.map(...) } }
Fix

Modified buildXmlObjectPropertyAssignments in buildXmlSerializerFunction.ts to read xmlOptions.itemsName and
xmlOptions.unwrapped for array properties and generate the correct nested structure.

Testing

• Added scenario tests for wrapped and unwrapped arrays with model items (10 tests)
• All 509 unit tests pass
• All 878 azure-modular integration tests pass

Fixes #3771