test(security): add zero-address rejection tests for mint and admin functions

[lmduy2612]

Description

Fixes #638

This PR adds comprehensive zero-address validation and unit test coverage across all contracts in the workspace:

• Credential NFT:
  • Validates recipient address in mint_credential against chainlearn_shared::zero_address, panicking with "cannot mint to zero address".
  • Added unit test test_mint_credential_to_zero_address_panics in tests/unit/credential_tests.rs.
  • Added unit test test_transfer_admin_to_zero_address_panics in tests/unit/credential_tests.rs.
• Progress Tracker:
  • Added unit test test_transfer_admin_to_zero_address_panics in tests/unit/progress_tests.rs.
  • Validates delegate address in delegate_progress, panicking with "cannot delegate to zero address".
  • Added unit test test_delegate_progress_to_zero_address_panics in tests/unit/progress_tests.rs.
• Learn Token:
  • Validates beneficiary address in create_vesting, panicking with "cannot vest to zero address".
  • Added unit test test_create_vesting_to_zero_address_panics in tests/unit/token_tests.rs.
  • Added unit test test_transfer_admin_to_zero_address_panics in tests/unit/token_tests.rs.

Verification

• cargo test --test credential_tests -- test_mint_credential_to_zero_address_panics (Passed)
• cargo test --test credential_tests -- test_transfer_admin_to_zero_address_panics (Passed)
• cargo test --test progress_tests -- test_transfer_admin_to_zero_address_panics (Passed)
• cargo test --test progress_tests -- test_delegate_progress_to_zero_address_panics (Passed)
• cargo test --test token_tests -- test_create_vesting_to_zero_address_panics (Passed)
• cargo test --test token_tests -- test_transfer_admin_to_zero_address_panics (Passed)
• Full suites pass: credential_tests (30/30) and progress_tests (108/108).