feat: add notification delivery idempotency

[charleswilliams788]

Overview

This PR adds Notification Delivery Idempotency to the hello-world contract so that repeated submission of the same notification request does not produce duplicate deliveries. Idempotency state is persisted on-chain, and concurrent duplicate requests are handled safely.

Related Issue

Changes

🔁 Idempotency State & Types

• [MODIFY] contract/contracts/hello-world/src/base/types.rs

  • Added the idempotency key/record types needed to persist delivery state for a notification request.

• [MODIFY] contract/contracts/hello-world/src/base/errors.rs

  • Added error variants for idempotency conflicts / invalid idempotency state.

⚙️ Delivery Logic

• [MODIFY] contract/contracts/hello-world/src/autoshare_logic.rs

  • Delivery path now checks persisted idempotency state before creating a delivery, so repeated requests with the same key do not create duplicates.
  • Persists idempotency state on successful delivery and short-circuits duplicate requests.
  • Guards against concurrent duplicate requests by recording state atomically within the delivery flow.

• [MODIFY] contract/contracts/hello-world/src/lib.rs

  • Wired up the idempotency-aware delivery entry point and storage accessors.

🧪 Tests

• [MODIFY] contract/contracts/hello-world/src/tests/notification_test.rs
  • Added coverage for repeated requests not creating duplicate deliveries, persisted idempotency state, and concurrent duplicate handling.

Verification Results

cargo test -p hello-world notification
✅ notification tests passed

Acceptance Criteria	Status
Repeated requests do not create duplicate deliveries	✅ Duplicate requests short-circuit on persisted idempotency state
Idempotency state is persisted	✅ State stored on-chain via contract storage
Concurrent duplicate requests are handled safely	✅ State recorded atomically within the delivery flow

Closes #843

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