Improve doc for MSTEST0049 and TestContext

[Evangelink]

Give example on how to use the cancellation token

Fixes #52340 and relates to #52341

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Internal previews

📄 File	🔗 Preview link
docs/core/testing/mstest-analyzers/mstest0049.md	MSTEST0049: Flow TestContext cancellation token
docs/core/testing/unit-testing-mstest-writing-tests-testcontext.md	MSTest TestContext

[Copilot]

Pull request overview

This PR updates the MSTest documentation to include concrete examples of flowing TestContext.CancellationToken in async tests, addressing MSTEST0049 guidance and improving discoverability from the TestContext article.

Changes:

• Add a Cancellation token section to the TestContext documentation, including snippet references for property-based access and constructor injection.
• Expand the MSTEST0049 analyzer page with before/after examples and suppression guidance.
• Add new C# snippet files and a snippet project to support the new documentation examples.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
docs/core/testing/unit-testing-mstest-writing-tests-testcontext.md	Adds a new section describing TestContext.CancellationToken and links to new snippets.
docs/core/testing/mstest-analyzers/mstest0049.md	Adds new examples demonstrating MSTEST0049 triggers and fixes, plus suppression instructions.
docs/core/testing/snippets/testcontext/csharp-cancellation/project.csproj	Introduces a new snippet project for the cancellation-token examples.
docs/core/testing/snippets/testcontext/csharp-cancellation/CancellationToken.cs	Adds a property-based TestContext.CancellationToken example.
docs/core/testing/snippets/testcontext/csharp-cancellation/CancellationTokenCtor.cs	Adds a constructor-injection example for accessing TestContext.CancellationToken.

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