Define role already granted

[d3v-active]

Closes #431

Added RoleAlreadyGranted Error Code: Introduced AdminError::RoleAlreadyGranted (= 7) to the AdminError enum inside the admin core module (contracts/admin/src/lib.rs). This clearly distinguishes between an invalid grant versus other failures.
Added Validation Check in Role Assignment: Integrated a new validation step within the internal _grant_role function. It now proactively checks has_role(env, role, address) and safely panics with RoleAlreadyGranted if the target address already holds the requested role.

Why this change was made
Previously, re-granting an already held role would overwrite the existing persistent storage state and extend the time-to-live unnecessarily without indicating to the caller that the role was already granted. This explicit failure ensures strict access control logic, optimizes gas by failing early, and guarantees that no redundant storage modifications are processed.

Acceptance Criteria met
Function logic operates securely: The state change is strictly guarded; the function aborts before interacting with the persistent ledger storage.
Events/Errors are properly defined/emitted: Added the specific error RoleAlreadyGranted alongside existing errors.
No unauthorized state modifications: Stops duplicate grants from being recorded or triggering duplicate grant events.

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