[Contract] Write Comprehensive Test Suite for All Contract Modules

[yusuftomilola]

A full test suite covering all contract modules using the Soroban SDK test environment. Tests should cover both happy paths and error cases.

Requirements

Modules to test:

• tests/mod.rs — shared test helpers, create_env(), mock addresses
• Core contract: initialisation, admin management, pause/unpause
• Asset: registration, validation failures, metadata update, ownership transfer, retirement
• Tokenization: tokenize, mint, burn, transfer, lock/unlock, ownership percentage, dividend distribution
• Voting: cast vote, tally, double-vote rejection, proposal pass check
• Transfer restrictions: whitelist add/remove, blocked transfer
• Detokenization: propose, vote, execute
• Insurance policies: full lifecycle (create → suspend → renew → cancel)
• Insurance claims: full lifecycle (file → review → approve → pay; reject → dispute)
• Leasing: create → return, create → cancel (before start), expire

Coverage targets:

• Every public function has at least one passing test
• Every Error variant has at least one test that triggers it
• Multi-step lifecycle flows tested end-to-end

Acceptance Criteria

• cargo test passes with zero failures
• All Error variants triggered in at least one test
• Tests are isolated (each test creates a fresh Env)