Kill the child when ExecuteAsync is cancelled, as Execute does [patch]

[matt-edmondson]

Fixes #53

Problem

When NativeCommandExecutor.ExecuteAsync was cancelled, it returned Operation was cancelled. and disposed the Process. Disposing doesn't end the child, so the command kept running and still performed its side effects. The synchronous Execute already kills the child on cancellation.

Change

• ExecuteAsync now catches OperationCanceledException inside the scope where the process exists, and calls TryKill(process) before returning Cancelled(). A token that is already cancelled before start still short-circuits as before.
• The outer OperationCanceledException handler is gone. Every await that can observe the token is now inside the inner try, so that handler could no longer be reached.
• TryKill uses Kill(entireProcessTree: true) on netcoreapp3.0+, so on both paths any commands the shell started are killed with it. It falls back to Kill() on netstandard2.1.

Tests

• CommandExecutor_Cancellation_Stops_The_Command runs sleep 10; touch <marker> through each path (ping / type nul on Windows). It cancels after 250 ms and asserts that each run returned well before the 10 s side effect was due. It then waits until the side effect would have happened and asserts that neither marker exists.
• The first version used a 2 s side effect. Once, on Windows CI, the cancellation was delivered only as the command finished, which made the kill look broken. The longer delay and the elapsed-time assertions make that case fail with an accurate message instead.
• Proven both ways. Without the new TryKill call in the async handler, the test fails on Assert.IsFalse(File.Exists(asyncMarker)). With the fix, it passes.
• Full suite on Linux passes. Release build of Essentials.CommandExecutors.Native is clean on all targets.

This merges cleanly with main after #99 (Unix argument quoting).

🤖 Generated with Claude Code

https://claude.ai/code/session_01D7wytU6jsZ1cH3f6grCTz5

[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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