refactor(cluster-handler): centralize defaulting constants for webhook preparation

This change extracts hardcoded default values (replica counts, template names) from the MultigresCluster controller logic into a dedicated `constants.go` file.

Key changes:
- Created `constants.go` to hold Level 4 defaults (e.g., DefaultEtcdReplicas=3).
- Updated `template_logic.go` and `multigrescluster_controller.go` to use these constants.
- Updated controller tests to assert against the constant rather than a magic number, ensuring tests remain robust if defaults change.
- Added code comments explicitly noting that these files should eventually move to a shared `pkg/multigres/defaults` package to support the future Mutating Webhook without circular dependencies.

This ensures that both the reconciliation loop and the future admission webhook will rely on a Single Source of Truth for defaulting logic.

Author: fernando-villalba

pkg/cluster-handler/controller/multigrescluster/constants.go

@@ -0,0 +1,21 @@
+package multigrescluster
+
+// NOTE: We may want to consider moving this and template_logic.go to a different package at some point.
+// These can be used by
+
+const (
+	// DefaultEtcdReplicas is the default number of replicas for the managed Etcd cluster if not specified.
+	DefaultEtcdReplicas int32 = 3
+
+	// DefaultAdminReplicas is the default number of replicas for the MultiAdmin deployment if not specified.
+	DefaultAdminReplicas int32 = 1
+
+	// FallbackCoreTemplate is the name of the template to look for if no specific template is referenced.
+	FallbackCoreTemplate = "default"
+
+	// FallbackCellTemplate is the name of the template to look for if no specific template is referenced.
+	FallbackCellTemplate = "default"
+
+	// FallbackShardTemplate is the name of the template to look for if no specific template is referenced.
+	FallbackShardTemplate = "default"
+)

pkg/cluster-handler/controller/multigrescluster/multigrescluster_controller.go

@@ -163,7 +163,7 @@ func (r *MultigresClusterReconciler) reconcileGlobalComponents(ctx context.Conte
 			},
 		}
 		if _, err := controllerutil.CreateOrUpdate(ctx, r.Client, ts, func() error {
-			replicas := int32(3)
+			replicas := DefaultEtcdReplicas
 			if topoSpec.Etcd.Replicas != nil {
 				replicas = *topoSpec.Etcd.Replicas
 			}
@@ -190,7 +190,7 @@ func (r *MultigresClusterReconciler) reconcileGlobalComponents(ctx context.Conte
 			},
 		}
 		if _, err := controllerutil.CreateOrUpdate(ctx, r.Client, deploy, func() error {
-			replicas := int32(1)
+			replicas := DefaultAdminReplicas
 			if multiAdminSpec.Replicas != nil {
 				replicas = *multiAdminSpec.Replicas
 			}

pkg/cluster-handler/controller/multigrescluster/template_logic.go

@@ -39,7 +39,7 @@ func (r *TemplateResolver) ResolveCoreTemplate(ctx context.Context, templateName
 		name = r.Defaults.CoreTemplate
 	}
 	if name == "" {
-		name = "default"
+		name = FallbackCoreTemplate
 		isImplicitFallback = true
 	}
 
@@ -67,7 +67,7 @@ func (r *TemplateResolver) ResolveCellTemplate(ctx context.Context, templateName
 		name = r.Defaults.CellTemplate
 	}
 	if name == "" {
-		name = "default"
+		name = FallbackCellTemplate
 		isImplicitFallback = true
 	}
 
@@ -94,7 +94,7 @@ func (r *TemplateResolver) ResolveShardTemplate(ctx context.Context, templateNam
 		name = r.Defaults.ShardTemplate
 	}
 	if name == "" {
-		name = "default"
+		name = FallbackShardTemplate
 		isImplicitFallback = true
 	}