[CI Flakiness] Fix MSB4216 task host failures on macOS and dotnet-watch test hangs

[mmitche]

Fix two major CI flakiness root causes:

1. MSB4216 task host failures on macOS - add DOTNET_HOST_PATH to Helix scripts
2. dotnet-watch test hangs - close stdin before kill, add exit timeout, reduce DCP timeouts

Baseline: 48/100 builds failed (48% failure rate)

[Copilot]

Pull request overview

Reduces CI flakiness in Helix by improving MSBuild task host resolution (MSB4216 on macOS) and preventing dotnet-watch tests from hanging indefinitely when processes fail to terminate.

Changes:

• Export DOTNET_HOST_PATH in Helix test runner scripts so MSBuild task hosts can reliably locate the dotnet host.
• Bound DCP/Aspire watch operation timeouts in WatchableApp to avoid multi-hour hangs.
• Improve process teardown in test utilities by closing stdin before killing and adding a bounded wait for process exit.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
test/Microsoft.DotNet.HotReload.Test.Utilities/WatchableApp.cs	Replaces “effectively infinite” DCP/Aspire timeouts with bounded (300s) values to avoid Helix-duration hangs.
test/Microsoft.DotNet.HotReload.Test.Utilities/AwaitableProcess.cs	Improves disposal reliability by closing stdin before kill and adding a 30s exit wait timeout.
build/RunTestsOnHelix.sh	Sets DOTNET_HOST_PATH for Helix runs (notably fixes MSB4216 on macOS task hosts).
build/RunTestsOnHelix.cmd	Same as above for Windows Helix runs.

Comments suppressed due to low confidence (1)

test/Microsoft.DotNet.HotReload.Test.Utilities/AwaitableProcess.cs:263

• If the 30s timeout elapses, DisposeAsync continues without ensuring _processExitAwaiter has completed. In that case WaitForProcessExitAsync() may later try to call _outputCompletionSource.Cancel() after _outputCompletionSource.Dispose() runs here, which can throw ObjectDisposedException and introduce new flakiness. Consider either (a) not disposing _outputCompletionSource until _processExitAwaiter has completed, or (b) making WaitForProcessExitAsync() tolerate _outputCompletionSource being disposed (e.g., catch ObjectDisposedException around Cancel()).

            Process.Dispose();

            _outputCompletionSource.Dispose();
        }

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