Extract disabledResult and evidenceEval helpers to reduce boilerplate

Add two shared helpers to source.go and apply them across all sources:
- disabledResult(id, name) replaces the repeated inline SourceResult literal
- evidenceEval(r, severity) replaces the identical Evaluate body in 6 sources

checker/dnsbl.go

@@ -93,9 +93,7 @@ var DefaultDNSBLZones = []DNSBLZone{
 func (s *dnsblSource) Query(ctx context.Context, domain, registered string, opts sdk.CheckerOptions) []SourceResult {
 	zones := zonesFromOptions(opts)
 	if registered == "" || len(zones) == 0 {
-		return []SourceResult{{
-			SourceID: s.ID(), SourceName: s.Name(), Enabled: false,
-		}}
+		return disabledResult(s.ID(), s.Name())
 	}
 
 	out := make([]SourceResult, len(zones))