Parse template when validating it (#49282)

pkg/services/ngalert/api/tooling/definitions/provisioning_templates.go

@@ -2,6 +2,7 @@ package definitions
 
 import (
 	"fmt"
+	"html/template"
 	"regexp"
 	"strings"
 
@@ -74,6 +75,11 @@ func (t *MessageTemplate) Validate() error {
 		return fmt.Errorf("template must have content")
 	}
 
+	_, err := template.New("").Parse(t.Template)
+	if err != nil {
+		return fmt.Errorf("invalid template: %w", err)
+	}
+
 	content := strings.TrimSpace(t.Template)
 	found, err := regexp.MatchString(`\{\{\s*define`, content)
 	if err != nil {