[contract] Add 3 Tests to leaderboard_tests.rs (Part 2)

[Olowodarey]

📋 Overview

Expand test coverage for the leaderboard module by adding 3 new tests (Part 2 of 2).

✅ Tasks

• Add 3 new test functions to contract/tests/leaderboard_tests.rs
• Follow existing test patterns and helper functions
• Verify all tests pass

🧪 New Tests (3 total)

Test 1: Leaderboard Season Transition

#[test]
fn test_leaderboard_season_transition() {
    // Add entries to season 1
    // Start season 2
    // Add entries to season 2
    // Verify season 1 entries are preserved
    // Verify season 2 entries are separate
}

Test 2: Leaderboard Rewards Distribution

#[test]
fn test_leaderboard_rewards_distribution() {
    // Add entries with scores
    // Distribute rewards to top N
    // Verify correct users receive rewards
    // Verify amounts are proportional to rank
}

Test 3: Empty Leaderboard Query

#[test]
fn test_get_leaderboard_empty_season() {
    // Query leaderboard for season with no entries
    // Should return empty list, not error
}

🔍 Verification

• All 3 tests written
• All tests pass
• Tests follow existing patterns
• No clippy warnings

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