getProcessExitCode/waitForProcess should throw ECHILD rather than return ExitSuccess for non-child process

https://www.man7.org/linux/man-pages/man2/wait.2.html

```
ECHILD (for waitpid() or waitid()) The process specified by pid
(waitpid()) or idtype and id (waitid()) does not exist or
is not a child of the calling process. (This can happen
for one's own child if the action for SIGCHLD is set to
SIG_IGN. See also the Linux Notes section about threads.)
```

In https://github.com/well-typed/haskell-debugger, it took me a long while to uncover a bug that turned out to be caused by `getProcessExitCode` incorrectly returning `Just ExitSuccess` for a handle of a non-child process.

I'm "unsafely" (i.e with .Internals) constructing the handle from the PID of a process that was not spawned/is not a child of the process calling `getProcessExitCode`. It should throw something like `ECHILD`, following the manpage for `wait`.

I suspect `waitForProcess` should also fail with `ECHILD` here.

I've attached the AI generated counter example program I was using to observe the incorrect behavior outside of the much more complicated `haskell-debugger`

[NonChildProcessHandleDemo.txt](https://github.com/user-attachments/files/26755074/NonChildProcessHandleDemo.txt)

Start by locating the implementations of getProcessExitCode and waitForProcess, then run the attached NonChildProcessHandleDemo.txt example to reproduce the behavior. Compare handling of a non-child process with the wait(2) ECHILD semantics; done means both functions no longer report ExitSuccess for that case.

Written by the indexing model from the issue text.