fix(elasticache-alpha): cannot import Redis 7 serverless cache (#35629)

### Issue # (if applicable)

N/A

### Reason for this change
The `ServerlessCache` class was missing the `CacheEngine.REDIS_7` case in the switch statement that determines the default port for imported caches. 

So importing a Redis 7 serverless cache causes an error:

> ValidationError: Unsupported cache engine: redis_7



### Description of changes
* Added missing `CacheEngine.REDIS_7` case** in the `fromServerlessCacheAttributes`  method's switch statement to ensure Redis 7 caches use the correct default port.
* Add unit tests for all supported versions.



### Describe any new or updated permissions being added
N/A



### Description of how you validated changes
Add unit tests


### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: mazyu36

packages/@aws-cdk/aws-elasticache-alpha/lib/serverless-cache.ts

@@ -329,6 +329,7 @@ export class ServerlessCache extends ServerlessCacheBase {
             case CacheEngine.VALKEY_7:
             case CacheEngine.VALKEY_8:
             case CacheEngine.REDIS_LATEST:
+            case CacheEngine.REDIS_7:
               // Document showing the default port
               // https://docs.aws.amazon.com/AmazonElastiCache/latest/dg/set-up.html#elasticache-install-grant-access-VPN
               defaultPort = ec2.Port.tcp(6379);

packages/@aws-cdk/aws-elasticache-alpha/test/serverless-cache.test.ts

@@ -20,6 +20,22 @@ describe('serverless cache', () => {
       expect(cache.serverlessCacheArn).toEqual(`arn:${stack.partition}:elasticache:${stack.region}:${stack.account}:serverlesscache/my-serverless-cache`);
     });
 
+    test.each([
+      [CacheEngine.VALKEY_LATEST],
+      [CacheEngine.VALKEY_8],
+      [CacheEngine.VALKEY_7],
+      [CacheEngine.REDIS_LATEST],
+      [CacheEngine.REDIS_7],
+      [CacheEngine.MEMCACHED_LATEST],
+    ])('import serverless cache for %s', (cacheEngine) => {
+      const cache = ServerlessCache.fromServerlessCacheAttributes(stack, 'ImportedCache', {
+        serverlessCacheName: 'my-serverless-cache',
+        engine: cacheEngine,
+      });
+
+      expect(cache.serverlessCacheArn).toEqual(`arn:${stack.partition}:elasticache:${stack.region}:${stack.account}:serverlesscache/my-serverless-cache`);
+    });
+
     test('create serverless cache with full props', () => {
       const key = new Key(stack, 'Key', {});
       const securityGroup = new SecurityGroup(stack, 'SecurityGroup', { vpc });
@@ -73,15 +89,24 @@ describe('serverless cache', () => {
       });
     });
 
-    test('create serverless cache with VALKEY_LATEST enigne', () => {
+    test.each([
+      [CacheEngine.VALKEY_LATEST, 'valkey', Match.absent()],
+      [CacheEngine.VALKEY_8, 'valkey', '8'],
+      [CacheEngine.VALKEY_7, 'valkey', '7'],
+      [CacheEngine.REDIS_LATEST, 'redis', Match.absent()],
+      [CacheEngine.REDIS_7, 'redis', '7'],
+      [CacheEngine.MEMCACHED_LATEST, 'memcached', Match.absent()],
+    ])('test serverless cache version for %s', (cacheEngine, engine, version) => {
       new ServerlessCache(stack, 'Cache', {
+        description: 'Serverless cache',
         vpc,
-        engine: CacheEngine.VALKEY_LATEST,
+        engine: cacheEngine,
       });
 
       Template.fromStack(stack).hasResourceProperties('AWS::ElastiCache::ServerlessCache', {
-        Engine: 'valkey',
-        MajorEngineVersion: Match.absent(),
+        Description: 'Serverless cache',
+        Engine: engine,
+        MajorEngineVersion: version,
       });
     });
   });