[SC-25] Write unit tests for tokenization, voting, and dividend modules

[yusuftomilola]

Overview

Write unit tests for the tokenization, voting, dividend, and restriction modules inside contracts/contrib/.

Location

All work must be inside contracts/contrib/src/tests/.

Acceptance Criteria

• Tests for tokenize_asset: success, already tokenized panic
• Tests for transfer_tokens: success, insufficient balance panic, blacklisted address panic
• Tests for cast_vote: success, double vote panic, below threshold panic
• Tests for distribute_dividends and claim_dividends: proportional distribution verified with multiple holders
• Tests for execute_detokenization: end-to-end flow from proposal to execution
• All tests pass with cargo test