fix shutdown tests

[rosebyte]

Fixes #129832

Summary

This change makes the functional test deployer more robust in Helix environments where dotnet is not on PATH and where process start can fail transiently.

• Resolve the default dotnet host from the active testhost layout (next to the shared framework) instead of assuming a global PATH entry.
• Keep the x86-on-x64 override behaviour unchanged by still resolving the architecture-specific host when required.
• Harden shutdown logic so cleanup does not throw when a Process object was created but never successfully started.
• Add retry logic for self-host process launch (3 attempts with short delay) to tolerate transient CI/Helix launch failures.
• Preserve existing startup signalling and error propagation semantics while improving diagnostics around failed launches.

Testing

• Existing shutdown functional tests (the target failing scenarios in CI)

Note

This PR description was generated with AI assistance.

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[Copilot]

Pull request overview

This PR updates the Microsoft.Extensions.Hosting functional-test deployment infrastructure to make shutdown-related tests more reliable on constrained/Helix environments by hardening process launch and shutdown handling.

Changes:

• Add a retry loop when launching the self-hosted test process, and surface early-exit failures more consistently.
• Resolve the dotnet muxer path relative to the currently running shared framework (with PATH fallback) for Helix environments that don’t provide a global dotnet.
• Avoid Process.HasExited throwing during shutdown/cleanup when the Process object was never successfully started.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/libraries/Microsoft.Extensions.Hosting/tests/FunctionalTests/IntegrationTesting/src/Deployers/SelfHostDeployer.cs	Adds start retry logic and reorders startup flow to reduce flaky failures from transient process-launch issues.
src/libraries/Microsoft.Extensions.Hosting/tests/FunctionalTests/IntegrationTesting/src/Deployers/ApplicationDeployer.cs	Improves dotnet muxer resolution for Helix and hardens shutdown checks against unstarted Process instances.

[jkotas]

Is this really the case?

RemoteExecutor is launching a ton of process in CI/Helix machines. It does not have a retry loop like this and we do not see a problems with that.

[rosebyte]

Hi, I'm trying to find out, the PR was generated by Copilot, please take the comments as WIP.