Add unit tests for missing learn-token query functions

[SrvFernandes]

Closes #505

Summary

Adds dedicated unit tests for 9 previously untested query functions in learn-token contract.

Changes

• Added comprehensive test cases in tests/unit/token_tests.rs covering:
  • get_claim_history with multiple claims and edge cases
  • get_storage_size verification after various operations
  • total_minted_to per-address verification
  • get_vesting_schedule direct query testing
  • permit_nonce increment verification
  • get_admins list verification
  • has_role checks for different roles
  • prune_expired_allowance removal verification
  • cleanup_expired_allowances behavior testing

Testing

All new tests pass with cargo test --lib. Tests verify both happy paths and edge cases.

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