System.Runtime.Serialization.Xml: throw SerializationException on malformed prefix

[imcarolwang]

When parsing an unfinished prefixed element ("<s:"), ReadObject previously surfaced ArgumentException (param "name"). Now it wraps the error in SerializationException per serializer contract.

Add regression test:

• DataContractSerializer.cs: DCS_ReadObject_MalformedPrefix_EmptyLocalName_ThrowsSerializationException

Fixes #1409.

[Copilot]

Pull Request Overview

This PR improves error handling in the XML serialization library by ensuring that malformed XML with invalid prefixes throws the expected SerializationException rather than leaking an ArgumentException. The fix addresses cases where malformed prefixed elements (like <s:) cause the XML parser to encounter empty local names.

• Wraps ArgumentException with SerializationException to maintain consistent error handling contract
• Adds exception handling in two methods: InternalIsStartObject and IsStartObjectHandleExceptions
• Includes regression test to verify the fix works correctly

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
XmlObjectSerializer.cs	Adds exception handling to wrap ArgumentException in SerializationException for two XML parsing methods
DataContractSerializer.cs	Adds regression test with malformed XML input and helper test types

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