Hierarchical Keyring: cold-cache stampede — N concurrent decrypts → N DynamoDB/KMS calls

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### Problem:

The Node Hierarchical Keyring doesn't de-dupe concurrent branch-key lookups. If I fire a lot of decrypts for the same branch key at once against a cold cache, they all miss the cache together (it's only filled after the keystore call returns), so each one hits the keystore on its own.

So instead of one lookup I get N DynamoDB `GetItem` + N KMS `Decrypt` calls. Easy to repro: `await Promise.all` of ~3000 decrypts for the same key version, and you see ~3000 keystore calls instead of 1. Encrypt has the same problem since it shares the same code path.

### Solution:

Add single-flight to `getBranchKeyMaterials`: on a miss, the first caller starts the keystore fetch and stores the in-flight promise (keyed by cache entry id); everyone else for the same key awaits that promise instead of starting their own. The entry is dropped once it settles, so the materials cache still owns caching and TTL, and a failed request isn't shared — the next call just retries.

### Out of scope:

The legacy caching CMM has the same gap but it's a separate path, so I'm not touching it here.

[//]: # (NOTE: If you believe this might be a security issue, please email aws-security@amazon.com instead of creating a GitHub issue. For more details, see the AWS Vulnerability Reporting Guide: https://aws.amazon.com/security/vulnerability-reporting/ )