This commit resolves access control issues in the voting system contracts and fixes all failing tests. Key changes include:

- Refine role-based access control by changing key functions from onlyAdmin to onlyOperator:
  - VotingCore: createPoll, addCandidate, finalizePoll
  - Authentication: generateVoterCodes, setVotingCoreContract
  - ZKPVerification: setVerificationKey, setVotingCoreContract
  - VotingFactory: upgradeContract

- Enhance AccessControl contract with trusted contracts mechanism:
  - Add trustedContracts mapping and management functions
  - Update onlyOperator modifier to allow calls from trusted contracts
  - Enable cross-contract interactions for factory upgrades

- Fix test implementation:
  - Properly assign operator roles to test signers
  - Register VotingFactory as trusted contract for core contracts
  - Update test assertions to match new error messages
  - Fix event emission order in Authentication contract

- Add missing functionality:
  - Implement getVersionCount in VotingFactory
  - Add interface declaration in IVotingFactory

These changes ensure proper access control enforcement while maintaining
flexibility for operators to manage polls and for the factory to handle
contract upgrades.