termio/exec: call waitpid in process exit callback

Fixes #4554

When a process exited on its own (we didn't kill it), we were not
calling waitpid. This commit adds a waitpid call in the event loop
process exit notification.

This happened because when an external exit happened we could call
`subprocess.externalExit` which tells our subprocess manager to NOT kill
the process (since its already dead). Unfortunately in this case it
means we also didn't call waitpid.

src/termio/Exec.zig

@@ -352,6 +352,15 @@ fn processExit(
     const execdata = &td.backend.exec;
     execdata.exited = true;
 
+    // The process exited but kqueue/epoll/io_uring don't reap
+    // the process so we have to call waitpid here. Since we're sure
+    // the process has exited (this callback is called) we call it
+    // without any WNOHANG or anything.
+    if (comptime builtin.os.tag != .windows) {
+        const result = std.posix.waitpid(execdata.process.pid, 0);
+        log.debug("child process exited. waitpid result={}", .{result});
+    }
+
     // Determine how long the process was running for.
     const runtime_ms: ?u64 = runtime: {
         const process_end = std.time.Instant.now() catch break :runtime null;