Ensure cancellation::none implementation does not allocate

[bghgary]

This is a fix on top of #26.

This pull request introduces a new standalone test executable for Windows platforms to detect memory leaks related to the cancellation::none() implementation. The test leverages _CrtDumpMemoryLeaks to ensure that the no-destroy pattern used in cancellation logic does not result in leaks, especially when heap allocations are involved. The test is only run in debug builds, and is integrated into the build and test system for Windows.

New memory leak detection test for cancellation logic:

• Added a standalone executable cancellation_leak_test that verifies cancellation::none() does not cause memory leaks by invoking _CrtDumpMemoryLeaks during test execution (Source/Windows.Test/Threading/CancellationMemoryLeakTest.cpp).
• Integrated the new test into the Windows build and test pipeline by updating CMakeLists.txt to build and run the executable as a CTest, conditional on Windows and test configuration.

[Copilot]

Pull request overview

This PR optimizes the cancellation class to prevent memory allocations in the cancellation::none() singleton. The none() singleton uses a no_destroy wrapper whose destructor never runs, which would cause any member allocations to leak in debug builds on some platforms. The solution wraps m_listeners in std::optional to defer allocation until a listener is actually added, which never occurs for the none() singleton.

Changes:

• Wrapped m_listeners member in std::optional to enable lazy initialization
• Updated all accesses to m_listeners to use optional's -> operator after checking or emplacing
• Made throw_if_cancellation_requested() const for improved const-correctness

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

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