Implement XmlSerializer end element handling workaround and tests

[StephenMolloy]

This pull request improves handling of XML fragments over streaming transports, particularly when deserializing multiple root elements. The changes add a workaround (enabled by default) to avoid unnecessary reads after a top-level end element, preventing timeouts or exceptions when more data is not immediately available. An AppContext opt-out is also included. The PR also adds comprehensive tests and supporting infrastructure to validate and control this behavior.

XML Serializer Compatibility and Behavior Improvements:

• Added a new UseXmlSerializerReadEndElementWorkaround switch (default: true) to LocalAppContextSwitches to control workaround behavior for reading end elements in XML fragments.
• Updated XmlSerializationReader.ReadEndElement() to respect the new switch, avoiding extra reads after a top-level end element when enabled, which prevents blocking or timeouts in streaming/fragment scenarios.
• Introduced AdvancePastTopLevelEndElementIfNeeded(XmlReader) utility and invoked it in XmlSerializer methods (GetMapping, CanDeserialize) to ensure correct reader advancement when the workaround is enabled. [1] [2] [3]

Testing and Infrastructure Enhancements:

• Added new tests to XmlSerializerTests.cs to verify deserialization of XML fragments with and without the workaround, including a test that disables the switch and expects a timeout exception.
• Implemented BlockingAfterBufferStream, a custom Stream that simulates blocking behavior after buffer exhaustion, to facilitate robust testing of streaming scenarios.
• Improved XmlSerializerAppContextSwitchScope to properly unset AppContext switches and clear cached values, ensuring test isolation and reliability.

Fixes: #47371

[Copilot]

Pull request overview

This PR improves XmlSerializer behavior when deserializing XML fragments over streaming transports by avoiding an extra read after completing a top-level end element (configurable via an AppContext switch), and adds tests to validate fragment deserialization behavior with the workaround enabled/disabled.

Changes:

• Added UseXmlSerializerReadEndElementWorkaround AppContext switch (default true) to control end-element read behavior.
• Updated XmlSerializationReader.ReadEndElement() and XmlSerializer entry points to avoid unnecessary reads after a top-level end element while still supporting multi-root fragment workflows.
• Added fragment/streaming-focused tests plus a custom stream and improved AppContext-switch test scoping.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/libraries/System.Private.Xml/src/System/Xml/Core/LocalAppContextSwitches.cs	Introduces the new AppContext switch (default enabled).
src/libraries/System.Private.Xml/src/System/Xml/Serialization/XmlSerializationReader.cs	Implements the end-element handling workaround in ReadEndElement().
src/libraries/System.Private.Xml/src/System/Xml/Serialization/XmlSerializer.cs	Advances past top-level end elements when starting Deserialize / CanDeserialize to enable multi-fragment reading.
src/libraries/System.Private.Xml/tests/XmlSerializer/XmlSerializerTests.cs	Adds streaming/fragment tests and supporting test infrastructure (custom stream + switch scope improvements).

[Copilot]

Remove the commented-out AppContext switch setup and make the test explicitly set the switch value (e.g., within an XmlSerializerAppContextSwitchScope). As written, the test both carries dead commented code and implicitly relies on the process-wide default/environment switch value, which can make the test less deterministic.

Suggested change

	XmlSerializer serializer = new XmlSerializer(typeof(SimpleType));
	byte[] data = Encoding.UTF8.GetBytes(payload);
	// Default should be true
	//const string switchName = "Switch.System.Xml.UseXmlSerializerReadEndElementWorkaround";
	//using var switchScope = new XmlSerializerAppContextSwitchScope(switchName, true);
	const string switchName = "Switch.System.Xml.UseXmlSerializerReadEndElementWorkaround";
	XmlSerializer serializer = new XmlSerializer(typeof(SimpleType));
	byte[] data = Encoding.UTF8.GetBytes(payload);
	using var switchScope = new XmlSerializerAppContextSwitchScope(switchName, true);

[Copilot]

BlockingAfterBufferStream does not actually block; once the buffer is exhausted it throws TimeoutException. Consider renaming it to reflect the behavior (timeout/throwing) or implementing actual blocking semantics to match the current name, to avoid misleading future readers of the test infrastructure.