fix(idempotency): keep payload validation hash on cache record completion

[svozza]

Summary

CachePersistenceLayer._updateRecord() rewrote the whole Redis/Valkey value with only status, expiry, and response data, dropping the payload validation hash written with the in-progress record. With payloadValidationJmesPath configured, an identical retry of a completed operation then failed with IdempotencyValidationError whenever the local cache did not hold the record. The completed record now keeps the validation attribute, matching the DynamoDB adapter.

Changes

• Serialize in-progress and completed cache records through a single private #encodeRecord() helper so both writes share one shape, including the validation attribute when payload validation is enabled
• Add a unit test asserting that a completed record written with payload validation retains its validation field

Issue number: closes #5705

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