[contract] Add 3 Tests to leaderboard_tests.rs (Part 1)

[Olowodarey]

📋 Overview

Expand test coverage for the leaderboard module by adding 3 new tests (Part 1 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 Ranking Order

#[test]
fn test_leaderboard_ranking_order() {
    // Add entries with different scores
    // Get leaderboard
    // Verify entries are sorted by score descending
}

Test 2: Leaderboard Pagination

#[test]
fn test_leaderboard_pagination() {
    // Add many entries
    // Get leaderboard with limit and offset
    // Verify correct subset is returned
}

Test 3: Update Existing Leaderboard Entry

#[test]
fn test_update_existing_leaderboard_entry() {
    // Add entry with score
    // Update same user's score
    // Verify score is updated, not duplicated
}

🔍 Verification

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

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