refactor: use unused constants instead of removing them

- Export unitTypes as UnitTypes and add HasValidUnitSuffix helper
- Use killed const (exit code 130) in execute() to detect SIGINT
- Update go.mod to go 1.26

go.mod

@@ -1,3 +1,3 @@
 module github.com/taigrr/systemctl
 
-go 1.21
+go 1.26

structs.go

@@ -1,5 +1,7 @@
 package systemctl
 
+import "strings"
+
 type Options struct {
 	UserMode bool
 }
@@ -11,3 +13,30 @@ type Unit struct {
 	Sub         string
 	Description string
 }
+
+// UnitTypes contains all valid systemd unit type suffixes.
+var UnitTypes = []string{
+	"automount",
+	"device",
+	"mount",
+	"path",
+	"scope",
+	"service",
+	"slice",
+	"snapshot",
+	"socket",
+	"swap",
+	"target",
+	"timer",
+}
+
+// HasValidUnitSuffix checks whether the given unit name ends with a valid
+// systemd unit type suffix (e.g. ".service", ".timer").
+func HasValidUnitSuffix(unit string) bool {
+	for _, t := range UnitTypes {
+		if strings.HasSuffix(unit, "."+t) {
+			return true
+		}
+	}
+	return false
+}

util.go

@@ -11,6 +11,9 @@ import (
 
 var systemctl string
 
+// killed is the exit code returned when a process is terminated by SIGINT.
+const killed = 130
+
 func init() {
 	path, _ := exec.LookPath("systemctl")
 	systemctl = path
@@ -37,6 +40,10 @@ func execute(ctx context.Context, args []string) (string, string, int, error) {
 	warnings = stderr.String()
 	code = cmd.ProcessState.ExitCode()
 
+	if code == killed {
+		return output, warnings, code, ErrExecTimeout
+	}
+
 	customErr := filterErr(warnings)
 	if customErr != nil {
 		err = customErr