[patch] Treat a partly failed process-tree kill as best effort, so Cancel() never throws

[matt-edmondson]

Fixes #107

Problem

Process.Kill(entireProcessTree: true) reports descendants it couldn't terminate as AggregateException, such as a sudo child or an elevated process. TryKill caught only InvalidOperationException, Win32Exception and NotSupportedException, so the exception escaped from all three call sites:

• The token registration: the caller's own cts.Cancel() threw, or with CancelAfter, the timer thread did.
• The OperationCanceledException catch: it replaced the documented OperationCanceledException.
• The failed-read catch: it hid the handler's real exception.

Change

• TryKill now also catches AggregateException. The runtime's tree kill carries on past each failure, so everything it could kill has already been killed.
• The seam the issue asked for: the kill itself moved into a private Kill(Process), which keeps the netstandard2.x single-process fallback. TryKill(Process) forwards to a new internal TryKill(Process, Action<Process> kill), so a test can inject a kill that fails. The XML doc says why this method must never throw.

Tests

New CancelShouldNotThrowWhenTheTreeKillCannotTerminateEveryDescendant:

• Starts a real sleep.
• Registers TryKill with a kill that throws the runtime's AggregateException(Win32Exception) on a token, the same way RunAsync registers its kill.
• Checks that cts.Cancel() returns normally, and that a direct TryKill call, as the catch blocks make it, doesn't throw either.

Checked locally (Linux, net10.0):

• With the new catch removed, the test fails with the exception coming out of CancellationTokenSource.ExecuteCallbackHandlers, which is the issue's cts.Cancel() symptom.
• With the change, the full suite passes: 58 passed, 2 skipped (the elevation tests). The library builds for all target frameworks with 0 errors.

Not tested end to end: a real process tree with an unkillable descendant. As the issue says, a sandboxed root environment can't easily create one. The seam test covers the exception path that such a tree produces.

Independent of #110 and #111, which touch other parts of RunAsync and AsyncProcessStreamReader.

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https://claude.ai/code/session_01Bm33TiYrofqKs4oGc3UGUU

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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