fix(ecs): route new namespace method via field

Merging main brought in addExistingDefaultCloudMapNamespace (#36812),
which reads and writes the private _defaultCloudMapNamespace backing
field that this branch replaces with a readonly
defaultCloudMapNamespace field written through
setDefaultCloudMapNamespace. The two sides touched different hunks so
git merged them without conflict, but the new method no longer compiled
(TS2551) and the build failed at jsii.

Route the new method through the field and its helper.

Author: laazyj

packages/aws-cdk-lib/aws-ecs/lib/cluster.ts

@@ -556,7 +556,7 @@ export class Cluster extends Resource implements ICluster {
    */
   @MethodMetadata()
   public addExistingDefaultCloudMapNamespace(options: ExistingCloudMapNamespaceOptions): cloudmap.INamespace {
-    if (this._defaultCloudMapNamespace !== undefined) {
+    if (this.defaultCloudMapNamespace !== undefined) {
       throw new ValidationError(lit`OnlyDefaultNamespaceOnce`, 'Can only add default namespace once.', this);
     }
 
@@ -585,7 +585,7 @@ export class Cluster extends Resource implements ICluster {
         'same VPC as this ECS cluster for service discovery to function correctly.');
     }
 
-    this._defaultCloudMapNamespace = options.namespace;
+    setDefaultCloudMapNamespace(this, options.namespace);
     if (options.useForServiceConnect) {
       // Validate ARN is well-formed for imported namespaces
       const nsArn = options.namespace.namespaceArn;