Validate from_address option via net/mail at the host

Reject malformed addresses up-front instead of letting them surface as
a runtime SMTP failure during Collect.

checker/provider.go

@@ -2,6 +2,9 @@ package checker
 
 import (
 	"encoding/json"
+	"fmt"
+	"net/mail"
+	"strings"
 	"time"
 
 	sdk "git.happydns.org/checker-sdk-go/checker"
@@ -21,6 +24,27 @@ func (p *happyDeliverProvider) Definition() *sdk.CheckerDefinition {
 	return Definition()
 }
 
+// ValidateOptions runs on the host before Collect so bad options are rejected
+// up-front rather than surfacing as a runtime SMTP failure.
+func (p *happyDeliverProvider) ValidateOptions(opts sdk.CheckerOptions) error {
+	raw, ok := opts["from_address"]
+	if !ok {
+		return nil
+	}
+	s, ok := raw.(string)
+	if !ok {
+		return fmt.Errorf("from_address must be a string")
+	}
+	s = strings.TrimSpace(s)
+	if s == "" {
+		return nil
+	}
+	if _, err := mail.ParseAddress(s); err != nil {
+		return fmt.Errorf("invalid from_address: %w", err)
+	}
+	return nil
+}
+
 func (p *happyDeliverProvider) ExtractMetrics(ctx sdk.ReportContext, collectedAt time.Time) ([]sdk.CheckMetric, error) {
 	var data HappyDeliverData
 	if err := json.Unmarshal(ctx.Data(), &data); err != nil {

checker/provider_test.go

@@ -86,8 +86,8 @@ func TestDefinitionShape(t *testing.T) {
 	if def.ID != "happydeliver" {
 		t.Errorf("ID = %q", def.ID)
 	}
-	if !def.Availability.ApplyToDomain {
-		t.Error("should apply to domain")
+	if !def.Availability.ApplyToService {
+		t.Error("should apply to service")
 	}
 	if !def.HasMetrics {
 		t.Error("HasMetrics should be true")