Remove exit pipe monitoring on Unix

[Copilot]

Removes exit pipe infrastructure from Unix process spawning and waiting. Exit pipe provided redundant process exit monitoring on platforms that already have kqueue (macOS/FreeBSD) or pidfd (Linux).

Changes

Managed layer (SafeChildProcessHandle.Unix.cs):

• Removed _exitPipeFd field and related buffer
• Updated constructor and all wait methods to eliminate exit pipe parameter
• Simplified ReleaseHandle() to only close pidfd

Native layer (pal_process.c):

• Removed exit pipe creation from spawn_process() in both posix_spawn and fork/exec paths
• Updated try_wait_for_exit(), try_wait_for_exit_cancellable(), and wait_for_exit_or_kill_on_timeout() to use only kqueue or pidfd
• Returns ENOTSUP on platforms without kqueue or pidfd support for wait operations with timeout/cancellation

Process monitoring after change

• macOS/FreeBSD: kqueue with EVFILT_PROC
• Linux: pidfd with poll
• Other Unix: ENOTSUP for timed/cancellable waits

Net reduction: 96 lines

Original prompt

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This section details on the original issue you should resolve

<issue_title>Remove all the logic related to "exit pipe" on Unix</issue_title>
<issue_description>- Remove all the code related to "exit pipe" on Unix:
- from the managed layer, including:

ProcessPlayground/Library/SafeChildProcessHandle.Unix.cs

Line 23 in f2220ca

private static readonly byte[] s_exitPipeBuffer = new byte[1];

ProcessPlayground/Library/SafeChildProcessHandle.Unix.cs

Line 25 in f2220ca

private readonly int _exitPipeFd;

ProcessPlayground/Library/SafeChildProcessHandle.Unix.cs

Line 118 in f2220ca

out int exitPipeFd,

- from the native layer, including:
- the spawn_process method:

ProcessPlayground/Library/native/pal_process.c

Line 159 in dfcd508

int* out_exit_pipe_fd,

ProcessPlayground/Library/native/pal_process.c

Lines 192 to 195 in dfcd508

	// Create pipe for exit monitoring (CLOEXEC to avoid other parallel processes inheriting it)
	if (create_cloexec_pipe(exit_pipe) != 0) {
	return -1;
	}

- for other native methods like try_wait_for_exit_cancellable, try_wait_for_exit, wait_for_exit_or_kill_on_timeout, remove the int exitPipeFd argument and when neither HAVE_KQUEUE or HAVE_PIDFD are defined, set errno = ENOTSUP; and return.</issue_description>

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

• Fixes Remove all the logic related to "exit pipe" on Unix #207

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[adamsitnik]

LGTM!