[CONTRACTS] Add Soroban contract tests for issue #5

[liangtovi-debug]

Closes #5

Summary

• replaced template test with Soroban contract tests for current BountyContract API
• added coverage for tip, create_bounty, and cancel_bounty token-balance behavior
• added duplicate-creation panic test to lock invariant for (issue_id, maintainer)
• enabled unit test module in lib.rs with #[cfg(test)] mod test;

Validation

• cd contracts/bounty && cargo test
• Result: 4 passed, 0 failed