types: make CheckTarget.String() unambiguous by always emitting all fields

The previous implementation skipped empty fields, which meant targets
differing only in which fields were populated could produce the same
string (e.g. {UserId:"A"} and {DomainId:"A"} both gave "A"). This
caused key collisions when the string was used in storage index keys.

checker/types.go

@@ -21,7 +21,6 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
-	"strings"
 	"time"
 )
 
@@ -66,19 +65,11 @@ func (t CheckTarget) Scope() CheckScopeType {
 	return CheckScopeUser
 }
 
-// String returns a stable string representation of the target.
+// String returns a stable, unambiguous string representation of the target.
+// All three fields are always present (even when empty) so that different
+// targets never produce the same string.
 func (t CheckTarget) String() string {
-	var parts []string
+	return t.UserId + "/" + t.DomainId + "/" + t.ServiceId
-	if t.UserId != "" {
-		parts = append(parts, t.UserId)
-	}
-	if t.DomainId != "" {
-		parts = append(parts, t.DomainId)
-	}
-	if t.ServiceId != "" {
-		parts = append(parts, t.ServiceId)
-	}
-	return strings.Join(parts, "/")
 }
 
 // CheckerAvailability declares on which scopes a checker can operate.

checker/types_test.go

@@ -148,10 +148,13 @@ func TestCheckTarget_String(t *testing.T) {
 		target CheckTarget
 		want   string
 	}{
-		{CheckTarget{}, ""},
+		{CheckTarget{}, "//"},
-		{CheckTarget{UserId: "u1"}, "u1"},
+		{CheckTarget{UserId: "u1"}, "u1//"},
-		{CheckTarget{UserId: "u1", DomainId: "d1"}, "u1/d1"},
+		{CheckTarget{UserId: "u1", DomainId: "d1"}, "u1/d1/"},
 		{CheckTarget{UserId: "u1", DomainId: "d1", ServiceId: "s1"}, "u1/d1/s1"},
+		// Ensure different targets with different empty fields don't collide.
+		{CheckTarget{DomainId: "d1"}, "/d1/"},
+		{CheckTarget{ServiceId: "s1"}, "//s1"},
 	}
 	for _, tt := range tests {
 		if got := tt.target.String(); got != tt.want {