Add DataContractSerializer tests for collection types

[Copilot]

This PR adds comprehensive tests for DataContractSerializer serialization of various collection types that were missing test coverage.

Tests Added

The following new test methods have been added to DataContractSerializerTests:

1. DCS_SortedList_IntSimpleType - Tests serialization of SortedList<int, SimpleType>
2. DCS_ObservableCollection_Int - Tests serialization of ObservableCollection<int>
3. DCS_ReadOnlyDictionary_IntString - Tests serialization of ReadOnlyDictionary<int, string>
4. DCS_ReadOnlyObservableCollection_Int - Tests serialization of ReadOnlyObservableCollection<int>
5. DCS_ListDictionary - Tests serialization of ListDictionary
6. DCS_StringCollection - Tests serialization of StringCollection

Implementation Details

• All tests follow the established patterns in the codebase using DataContractSerializerHelper.SerializeAndDeserialize
• Tests use skipStringCompare: true for robust serialization verification
• Each test verifies both successful serialization/deserialization and data integrity
• Collection contents are validated to ensure proper round-trip behavior

Test Results

• Before: 343 tests passing
• After: 349 tests passing (6 new tests added)
• All existing tests continue to pass, ensuring no regressions

The tests ensure that these commonly used collection types can be properly serialized and deserialized using DataContractSerializer, improving the overall test coverage for the serialization framework.

Fixes #24355.

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