feat: migrate state variables in access control contracts with tests …

[mgtmdccix-oss]

Closes #99

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Summary
Migrates state variables in contracts/access_control to improve robustness, security, and performance of the Kora Protocol.

Changes
Storage key migration
Split the single DataKey enum into two purpose-specific enums:

InstanceKey (Admin, Paused) — instance storage for contract-level config tied to the instance lifetime
PersistentKey (Role(Address)) — persistent storage for per-address role data with independently managed TTLs
This makes the storage layout explicit and prevents accidental misuse of storage tiers.

Instance storage TTL management
Added bump_instance() calling env.storage().instance().extend_ttl(...) on every state-mutating function. Previously, instance storage TTL was never extended, risking expiry of the Admin and Paused keys on a live network.

read_paused() helper
Extracted the repeated paused-flag read into a single private method used by pause, unpause, and is_paused.

Duplicate test block removed and extended
The old file had two mod tests blocks. Consolidated into one comprehensive suite and added 4 new tests that specifically validate the storage key migration:

test_instance_key_admin_and_paused_are_independent
test_persistent_key_role_entries_are_independent
test_no_role_none_tombstone_after_revoke
test_transfer_admin_removes_old_role_entry
Checklist
Compatible with existing Soroban infrastructure
Safe arithmetic (no silent overflows; no arithmetic in this contract)
Scoped to contracts/access_control only — no cross-contract changes
Input validation unchanged and robust
Comprehensive tests covering all edge cases

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