(ecs): enableDeploymentAlarms does not actually add additional alarms

### Describe the bug

`BaseService.enableDeploymentAlarms()` doesn't actually add new alarms to the deployment configuration.

### Regression Issue

- [ ] Select this option if this issue appears to be a regression.

### Last Known Working CDK Library Version

_No response_

### Expected Behavior

`BaseService.enableDeploymentAlarms()` should add the alarms to the deployment configuration

### Current Behavior

Any alarms passed to `BaseService.enableDeploymentAlarms()` are silently ignored.

### Reproduction Steps

```typescript
describe('enableDeploymentAlarms', () => {
test('enableDeploymentAlarms will concat alarms in the deployment configuration when called more than once', () => {
// GIVEN
const vpc = new ec2.Vpc(stack, 'Vpc');
const cluster = new ecs.Cluster(stack, 'EcsCluster', { vpc });
const taskDefinition = new ecs.FargateTaskDefinition(stack, 'FargateTaskDef');
taskDefinition.addContainer('web', {
image: ecs.ContainerImage.fromRegistry('amazon/amazon-ecs-sample'),
});

const expectedAlarmNames = ['alarm1', 'alarm2', 'alarm3'];

// WHEN
const service = new ecs.FargateService(stack, 'FargateService', {
cluster,
taskDefinition,
minHealthyPercent: 100,
maxHealthyPercent: 200,
deploymentAlarms: { alarmNames: ['alarm1'] },
});

service.enableDeploymentAlarms(['alarm2']);
service.enableDeploymentAlarms(['alarm3']);

// THEN
Template.fromStack(stack).hasResourceProperties('AWS::ECS::Service', {
DeploymentConfiguration: {
Alarms: {
AlarmNames: expectedAlarmNames,
Enable: true,
Rollback: true,
},
},
});
});
});
});
```

### Possible Solution

_No response_

### Additional Information/Context

_No response_

### AWS CDK Library version (aws-cdk-lib)

main

### AWS CDK CLI version

main

### Node.js Version

v20.19.5

### OS

Linux 2dc632a883eb 6.12.54-linuxkit #1 SMP Tue Nov 4 21:21:47 UTC 2025 aarch64 GNU/Linux

### Language

TypeScript

### Language Version

_No response_

### Other information

_No response_

Start at BaseService.enableDeploymentAlarms() and use the provided ecs test as the reproduction. Check the synthesized AWS::ECS::Service DeploymentConfiguration after calling the method more than once. Done means alarm1, alarm2, and alarm3 all appear in AlarmNames with Enable and Rollback still true.

Written by the indexing model from the issue text.