Handle CDATA in unknown extension data (IExtensibleDataObject)

[StephenMolloy]

DataContractSerializer threw a SerializationException when deserializing an unknown element whose value is carried in a CDATA section into ExtensionData (types implementing IExtensibleDataObject). ReadExtensionDataValue's NodeType switch handled Text/Element/EndElement but not CDATA, and MoveToContent stops on CDATA rather than folding it into Text, so the value hit the default case and threw. ReadUnknownXmlData similarly special-cased only Text and would drop leading CDATA content.

Treat CDATA like Text in both places. The content is captured as a string node and replays as escaped text on re-serialization, preserving round-trip semantics.

Fixes #59123

[Copilot]

Pull request overview

This PR fixes DataContractSerializer extension-data deserialization for unknown members whose element content is carried in a CDATA section by treating XmlNodeType.CDATA the same as XmlNodeType.Text during extension-data parsing, and adds a regression test to validate down-level round-tripping via IExtensibleDataObject.

Changes:

• Handle XmlNodeType.CDATA in ReadExtensionDataValue so primitive extension data can be read from CDATA content.
• Preserve leading CDATA content in ReadUnknownXmlData by recognizing CDATA as text content (instead of advancing past it).
• Add a regression test and simple versioned contracts to validate ExtensionData capture and round-trip behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/XmlObjectSerializerReadContext.cs	Treat CDATA as text when reading primitive extension data and when capturing unknown XML into XmlDataNode, avoiding exceptions/dropped content.
src/libraries/System.Runtime.Serialization.Xml/tests/DataContractSerializer.cs	Adds an xUnit regression test that exercises unknown-member-with-CDATA flowing into ExtensionData and round-tripping back into a newer contract.
src/libraries/System.Runtime.Serialization.Xml/tests/SerializationTypes.RuntimeOnly.cs	Adds CdataVersionedV1/V2 contracts implementing IExtensibleDataObject for the new regression test scenario.