Refactor LeetCode 1669 splice logic

[eselyavka]

Summary

• replace the previous index-tracking implementation in solution_01669.py
• use a direct pointer-splice approach to find the node before a, the node after b, and the tail of list2
• keep the existing unittest coverage unchanged

Testing

• python3 practice/leetcode/solutions_01500/solution_01669.py
• pre-commit hook: pytest
• pre-commit hook: pylint on changed Python files