Implement SafeProcessHandle.Open API

[Copilot]

Description

Implements SafeProcessHandle.Open(int processId) — opens an existing process by PID and returns a SafeProcessHandle.

• Windows: Uses OpenProcess with PROCESS_QUERY_LIMITED_INFORMATION | SYNCHRONIZE | PROCESS_TERMINATE.
• Linux: Uses pidfd_open syscall when available, falls back to kill(pid, 0).
• Other Unix: Uses kill(pid, 0) to verify process existence and signal permission.

using SafeProcessHandle handle = SafeProcessHandle.Open(pid);
handle.Kill();

Changes

• Ref assembly (ref/System.Diagnostics.Process.cs): Added Open declaration with platform attributes
• Shared (SafeProcessHandle.cs): Open method with argument validation, XML docs, [UnsupportedOSPlatform]/[SupportedOSPlatform] attributes
• Windows (SafeProcessHandle.Windows.cs): OpenCore via Interop.Kernel32.OpenProcess
• Unix (SafeProcessHandle.Unix.cs): OpenCore via Interop.Sys.OpenProcess, added _pidfd field with cleanup in ReleaseHandle, new private constructor for pidfd-backed handles. ForkAndExecProcess now also populates _pidfd so child processes started via Start() get a pidfd on Linux.
• Native (pal_process.c/h): Extracted TryOpenPidfd as a reusable linux-specific helper for pidfd_open. Extended SystemNative_ForkAndExecProcess with int32_t* outPidfd parameter — populated via TryOpenPidfd on the fork path (Linux), set to -1 on the posix_spawn path (macOS). SystemNative_OpenProcess also reuses the same helper. Added SYS_pidfd_open define (matching __NR_close_range pattern), WASI stub, entrypoint registration.
• Interop (Interop.OpenProcess.cs): New Unix P/Invoke for SystemNative_OpenProcess
• Interop (Interop.ForkAndExecProcess.cs): Extended P/Invoke and managed wrapper with out int parameter (matching the out int lpChildPid pattern) to read pidfd from native layer
• Tests (SafeProcessHandleTests.cs): Argument validation, non-existent PID, open running process, open-then-kill

Testing

• All 405 process tests pass (0 failures, 5 skipped)

[adamsitnik]

@copilot give my feedback a try. Run all process tests and ensure they pass before you push any changes.

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[Copilot]

@copilot give my feedback a try. Run all process tests and ensure they pass before you push any changes.

Done in e2bc39d. All 405 process tests pass (0 failures, 5 skipped).

[adamsitnik]

@copilot address my feedback

[Copilot]

@copilot address my feedback

All three review items addressed in a114b09. All 405 process tests pass (0 failures, 5 skipped).