feat(rotators): AWS Secrets Manager rotator with SigV4 signer + secrets client
SigV4 implementation per AWS reference: canonical request -> string-to-sign
-> HMAC-derived signing key (kSecret -> kDate -> kRegion -> kService ->
kSigning) -> HMAC-SHA256 signature. Includes session token (STS) support.
SecretsManagerClient wraps PutSecretValue, GetSecretValue,
UpdateSecretVersionStage with custom endpoint support (LocalStack).
AWS API errors surface as AwsApiError carrying HTTP status + AWS __type.
AwsSecretsRotator implements 4-step contract:
- generate: PutSecretValue with AWSPENDING stage, captures version_id
- apply: no-op (PutSecretValue already exposed it)
- verify: GetSecretValue by version_id, byte-equal SecretString check
- commit: UpdateSecretVersionStage move AWSCURRENT to new + remove from old
- rollback_apply: UpdateSecretVersionStage remove AWSPENDING from new version
13 unit specs verify SigV4 idempotence + format, client methods (with
WebMock), rotator's full 4-step path, verify-mismatch, rollback_apply,
and can_rotate? gating.
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