Add batch of 15 missing LeetCode problems

[shravnchandr]

Generated: 15 new problems (e.g., _01_matrix, _3sum, accounts_merge) added to leetcode/.
Verification: All lint checks passed (black, isort, ruff, mypy).
Tests for new problems are valid and collected successfully (failing assertions as expected).

[wislertt]

Thank you for your contribution, @shravnchandr. However, several CI checks have failed, and there are a number of issues in this PR. For example, _3sum duplicates the existing three_sum problem.

Additionally, existing problems should not be modified within this PR. If you need to fix an existing problem, please open a separate PR for that.

To make the review process smoother, please consider starting with a smaller PR. Perhaps adding just one or a few problems at a time.

[shravnchandr]

Hi @wislertt, apologies for the error. As per your suggestion, I will start with smaller PRs. Thank you.