[codex] Add optimized diagonal traverse solution

[eselyavka]

Summary

• Add Solution2 for LeetCode 498 using direct diagonal direction simulation.
• Expand in-file tests to validate both implementations across square, rectangular, and single-cell matrices.

Validation

• python3 -m unittest practice.leetcode.solutions_00300.solution_00498
• Commit hook: 805 passed in 1.23s
• Commit hook pylint on changed Python files: 10.00/10