refactor(controller): dedupe the terminal-error annotation decoder

decodeTerminalErrorAnnotation duplicated decodeTerminalError (both born in the
same commit). Route the one caller to decodeTerminalError and delete the copy;
the caller already guards against an empty annotation, so the canonical
decoder's empty-string handling is equivalent here.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: scotwells

internal/controller/instance_controller.go

@@ -657,7 +657,7 @@ func (r *InstanceReconciler) applyTerminalErrorFromWD(
 ) (changed, terminal bool) {
 	// Path 1: annotation (federation-safe).
 	if termErrRaw := wd.Annotations[computev1alpha.ReferencedDataErrorAnnotation]; termErrRaw != "" {
-		termReason, termMessage, decodeErr := decodeTerminalErrorAnnotation(termErrRaw)
+		termReason, termMessage, decodeErr := decodeTerminalError(termErrRaw)
 		if decodeErr != nil {
 			log.FromContext(ctx).V(1).Info("malformed referenced-data-error annotation on WD; ignoring",
 				"workloadDeployment", wd.Name, "error", decodeErr)
@@ -679,21 +679,6 @@ func (r *InstanceReconciler) applyTerminalErrorFromWD(
 	return false, false
 }
 
-// decodeTerminalErrorAnnotation parses the value of ReferencedDataErrorAnnotation
-// into (reason, message). Returns an error when the annotation is malformed.
-// Both files in the same package share this unexported helper through Go's
-// package-level visibility — no cross-file coupling issue.
-func decodeTerminalErrorAnnotation(raw string) (reason, message string, err error) {
-	var payload struct {
-		Reason  string `json:"reason"`
-		Message string `json:"message"`
-	}
-	if err := json.Unmarshal([]byte(raw), &payload); err != nil {
-		return "", "", fmt.Errorf("decode referenced-data-error annotation %q: %w", raw, err)
-	}
-	return payload.Reason, payload.Message, nil
-}
-
 // listPresentCompanionsByKindName returns a set keyed by kind-qualified tokens
 // ("Kind/name", e.g. "ConfigMap/app-config") for every companion ConfigMap and
 // Secret present in the given namespace (matched by ReferencedDataLabel). This