[macOS arm64] dotnet-watch cleanup hangs in Process.Kill(entireProcessTree: true)

## Build Information

Build: https://dev.azure.com/dnceng-public/public/_build/results?buildId=1542039
Leg Name: Mac arm64 - osx.15.arm64.open
Work Item: dotnet-watch.Tests.dll.2

## Error Details

During test cleanup, multiple calls to `Process.Kill(entireProcessTree: true)` do not return. Nine processes logged entry into the kill operation, but only three logged completion. Helix eventually collected hang dumps and terminated the test process.

This appeared after the runtime flow in https://github.com/dotnet/sdk/pull/55595 containing [dotnet/runtime#128598](https://github.com/dotnet/runtime/pull/128598), which changed Unix process-tree termination behavior.

Confidence: High that the immediate hang occurs inside `Process.Kill(entireProcessTree: true)`; moderate that runtime PR #128598 introduced it.

```text
[TEST AwaitableProcess.cs:237] Killing process tree for process 18251
Hang dump timeout of '00:48:00' expired
Test application process didn't exit gracefully, exit code is '137'
```

## Error Message

```json
{
"ErrorPattern": "Work item 'dotnet-watch\\.Tests\\.dll\\.2' in job 'Mac arm64 - osx\\.15\\.arm64\\.open \\([0-9a-f-]+\\)' failed \\(Finished, exit code 7\\)\\.",
"BuildRetry": false,
"ExcludeConsoleLog": false
}
```

### Known issue validation
**Build: :mag_right:** https://dev.azure.com/dnceng-public/public/_build/results?buildId=1542039
**Error message validated:** `[\[TEST AwaitableProcess\.cs:\d+\] Killing process tree for process \d+ Hang dump timeout of '00:48:00' expired Test application process didn't exit gracefully, exit code is '137'`]
**Result validation:** :x: Known issue did not match with the provided build.
**Validation performed at:** 8/6/2026 1:38:19 PM UTC

### Report
#### Summary
|24-Hour Hit Count|7-Day Hit Count|1-Month Count|
|---|---|---|
|0|0|0|

### Known issue validation
**Build: :mag_right:** https://dev.azure.com/dnceng-public/public/_build/results?buildId=1542039
**Error message validated:** `[Work item 'dotnet-watch\.Tests\.dll\.2' in job 'Mac arm64 - osx\.15\.arm64\.open \([0-9a-f-]+\)' failed \(Finished, exit code 7\)\.`]
**Result validation:** :x: Known issue did not match with the provided build.
**Validation performed at:** 8/7/2026 1:27:13 PM UTC

### Report
#### Summary
|24-Hour Hit Count|7-Day Hit Count|1-Month Count|
|---|---|---|
|0|0|0|

### Report
#### Summary
|24-Hour Hit Count|7-Day Hit Count|1-Month Count|
|---|---|---|
|0|0|0|