Add edge case unit tests for leetcode module

[branhoff]

We need to improve our test coverage for the LeetCode module by adding unit tests for various edge cases. This will help ensure the robustness of our command under different scenarios.

Tasks:

1. Add a test for empty username input
2. Add a test for very long usernames
3. Add a test for usernames with special characters
4. Add a test for when the API returns unexpected data formats

For each test, follow the established pattern:

• Use @pytest.mark.asyncio decorator
• Use AsyncMock() for the context
• Patch the fetch_leetcode_profile method
• Call the command callback directly: await cog.leetcode.callback(cog, ctx, username)
• Assert the expected behavior

Example structure for each test:

@pytest.mark.asyncio
async def test_leetcode_[scenario_name](cog):
    ctx = AsyncMock()
    username = [appropriate_test_username]

    with patch.object(cog, 'fetch_leetcode_profile') as mock_fetch:
        mock_fetch.return_value = [appropriate_mock_return]

        await cog.leetcode.callback(cog, ctx, username)

        # Assert expected behavior

Definition of Done:

• All listed edge case tests are implemented
• Tests pass and provide meaningful feedback on failure
• Code is reviewed and approved by at least one other team member
• Test coverage for the leecode module is increased