Implement ProcessStartOptions.CreateSuspended and SafeChildProcessHandle.Resume

[Copilot]

• Add CreateSuspended property to ProcessStartOptions
• Add Resume() method to SafeChildProcessHandle
• Implement Windows support (CREATE_SUSPENDED flag, store thread handle, ResumeThread)
• Implement Unix/Linux support (resume pipe, blocking read, write byte to resume)
• Add Interop declarations for Windows ResumeThread API
• Add tests for CreateSuspended and Resume functionality (13 tests)
• Address review feedback: Use DirectoryInfo.FullName for path comparison
• Resolve merge conflicts with main branch (SendSignal API)
• All 85 tests pass (81 original + 4 from main)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Implement ProcessStartOptions.CreateSuspended and SafeChildProcessHandle.Resume</issue_title>
<issue_description>On Windows:

• Use CREATE_SUSPENDED flag in CreateProcess
• Stores thread handle for suspended processes
• Resume() calls ResumeThread and releases thread handle

Unix/Linux (pipe-based approach):

• Parent creates a resume pipe before forking (with CLOEXEC)
• Child performs blocking read() from resume pipe before execve()
• Parent returns with resume pipe write end
• Resume() writes a single byte to the pipe to unblock the child
• Uses regular fork() instead of vfork() when suspended to avoid parent blocking
• Skips wait pipe synchronization when suspended to avoid deadlock</issue_description>

<agent_instructions>Add some tests</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Implement ProcessStartOptions.CreateSuspended and SafeChildProcessHandle.Resume #71

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.

[adamsitnik]

@copilot overall it LGTM, but we need to fix one test failure PTAL at my comments

[adamsitnik]

I think it's better to do the following:

• store the DirectoryInfo returned by Directory.CreateDirectory(tempDir);
• use it's FullName as expected output for the assert

[Copilot]

Fixed in c69f2fd. Now storing the DirectoryInfo returned by Directory.CreateDirectory() and using its FullName property for the expected path comparison.

[adamsitnik]

@copilot solve the merge conflicts

[Copilot]

@copilot solve the merge conflicts

Merge conflicts resolved in 0c7feab. Successfully merged main branch which added SendSignal API alongside the CreateSuspended/Resume implementation. All 85 tests pass.