AWS ElastiCache: open real-cloud parity gaps #1472

cloudemu's ElastiCache emulation still differs from real AWS in the ways listed below. Each item is a separate fix, so pick any one and open a PR for it against development. Comment here with the item you're taking so two people don't work on the same one.

Check the expected behaviour against the AWS API reference and the real CLI or SDK. A good fix goes in the provider (providers/aws/...) so the Go library and cloudemu serve behave the same, and adds a test that fails before the fix. See CONTRIBUTING.md for setup.

High

• CreateReplicationGroup NumNodeGroups/ReplicasPerNodeGroup/NodeGroupConfiguration silently dropped (single shard) (no NumNodeGroups parsing in server/aws/elasticache)
• ModifyReplicationGroupShardConfiguration missing

Medium

• ModifyCacheSubnetGroup missing
• TestFailover missing
• RBAC users/user groups family missing
• Serverless cache family missing
• DescribeCacheClusters ignores MaxRecords/Marker pagination
• Missing CloudWatch metrics: replace fake command-name metrics with real AWS/ElastiCache set

Low

• Global Datastore family missing
• DescribeServiceUpdates/DescribeUpdateActions/BatchApplyUpdateAction missing
• Legacy CacheSecurityGroup + Reserved Node APIs missing
• DescribeEvents answered by RDS handler (RDS xmlns; empty)
• Metrics essentially absent (only Reboots; no CPU/CacheHits/Evictions/CurrConnections)