fix(ecs): remove unnecessary error when adding volume to external task definition (#19774)

fixes #19259

----

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Author: peterwoodworth

packages/@aws-cdk/aws-ecs/lib/external/external-task-definition.ts

@@ -8,7 +8,6 @@ import {
   ITaskDefinition,
   NetworkMode,
   TaskDefinition,
-  Volume,
 } from '../base/task-definition';
 
 /**
@@ -75,13 +74,6 @@ export class ExternalTaskDefinition extends TaskDefinition implements IExternalT
     });
   }
 
-  /**
-   * Overridden method to throw error, as volumes are not supported for external task definitions
-   */
-  public addVolume(_volume: Volume) {
-    throw new Error('External task definitions doesnt support volumes');
-  }
-
   /**
    * Overriden method to throw error as interface accelerators are not supported for external tasks
    */

packages/@aws-cdk/aws-ecs/test/external/external-task-definition.test.ts

@@ -601,17 +601,30 @@ describe('external task definition', () => {
     Annotations.fromStack(stack).hasWarning('/Default/ExternalTaskDef/web', "Proper policies need to be attached before pulling from ECR repository, or use 'fromEcrRepository'.");
   });
 
-  test('correctly sets volumes from', () => {
+  test('correctly sets volumes', () => {
+    // GIVEN
     const stack = new cdk.Stack();
     const taskDefinition = new ecs.ExternalTaskDefinition(stack, 'ExternalTaskDef', {});
 
-    // THEN
-    expect(() => taskDefinition.addVolume({
+    // WHEN
+    taskDefinition.addVolume({
       host: {
         sourcePath: '/tmp/cache',
       },
       name: 'scratch',
-    })).toThrow('External task definitions doesnt support volumes' );
+    });
+
+    // THEN
+    Template.fromStack(stack).hasResourceProperties('AWS::ECS::TaskDefinition', {
+      Volumes: [
+        {
+          Host: {
+            SourcePath: '/tmp/cache',
+          },
+          Name: 'scratch',
+        },
+      ],
+    });
   });
 
   test('error when interferenceAccelerators set', () => {