feat(contracts): implement notification revocation mechanism and auth safeguards (#176)

[Abdullahi-Code9]

Closes #176

Summary

Addresses #176 by adding a secure notification revocation mechanism to the core smart contract layer. This feature allows authorized entities (the notification creator or the contract admin) to permanently invalidate scheduled notifications before a recipient interacts with them, keeping a transparent on-chain history for auditing.

Key Changes

• State Extension: Updated ScheduledNotification in types.rs to track revocation vectors via revoked_by: Option<Address> and revoked_at: Option<u64>.
• Access Control & Safety: Introduced strict authorization checks in autoshare_logic.rs. Unauthorized revoke attempts trigger a custom Error::NotAuthorizedToRevoke error code.
• State Locks: Prevented any downstream interactions (cancellations or expirations) on already-revoked notifications by throwing Error::NotificationRevoked.
• Event-Driven Framework: Added the high-priority NotificationRevoked event matrix to events.rs so that downstream indexers can efficiently detect revocations.
• Testing Matrix: Extended revocation_test.rs to safely include extern crate std; for test boundaries, running 14 comprehensive functional scenarios capturing execution paths, edge cases, and permission overrides.

Verification Results

Verified implementation boundaries and event emissions against structural state changes via local Cargo framework.
cargo test revocation
Result: 14 passed, 0 failed.

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