Writing Mstest Tests

Write new MSTest unit tests using MSTest 3.x/4.x APIs.
Modernize existing tests by replacing generic assertions with specific `Assert` APIs.
Replace `[ExpectedException]` with `Assert.Throws` or `Assert.ThrowsExactly`.
Fix `MSTESTxxxx` analyzer diagnostics in MSTest tests.

Not for test quality audits, anti-pattern detection, or flaky-test investigation.
Not for running or executing tests.
Not for MSTest version migration.
Not for CI/CD pipeline configuration.
Not for xUnit, NUnit, TUnit, or non-.NET languages.
Check the test project for its MSTest version and configuration.
Check that `Assert.AreEqual` uses expected first and actual second.
Check that timeout-bound async tests flow `TestContext.CancellationToken` into awaited calls.

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