Implement atomic request id deduplication

[Osvaldo Andrade (osvaldoandrade)]

Summary

• add optional request_id to AtomicUpdateRequest and client atomic options
• cache successful atomic update results for retry dedup inside the Pebble shard for the 60s in-memory window
• preserve legacy at-least-once semantics when request_id is empty

Validation

• git diff --check
• go test ./internal/storage/adapter/pebble ./internal/server ./pkg/client ./pkg/protocol
• go test ./...

Closes #517