Update TestSTSSessionClaimsToSessionInfoCredentialGeneration to properly verify
deterministic credential generation:
- Remove misleading comment about 'randomness' - parts of credentials ARE deterministic
- Add assertions that AccessKeyId is identical for same SessionId (hash-based, deterministic)
- Add assertions that SessionToken is identical for same SessionId (hash-based, deterministic)
- Verify Expiration matches when SessionId is identical
- Document that SecretAccessKey is NOT deterministic (uses random.Read)
- Truncate expiresAt to second precision to avoid timing issues
This test now properly verifies that the deterministic components of credential
generation work correctly while acknowledging the cryptographic randomness of
the secret access key.