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).