[INFRA] Improve pull request merge tool

[pan3793]

Why are the changes needed?

This ports several useful improvements from Spark's merge tool, adapted to Kyuubi's GitHub-only workflow. It makes merges and backports safer, more traceable, and less error-prone for committers.

• Suggest backport branches in descending release order, such as branch-1.12, then branch-1.11, and skip branches already picked.
• Post a merge summary comment listing every branch and full commit link where the change landed.
• Normalize confirmation prompts to (y/N).
• Detect already-merged PRs more robustly, including reopened PRs and merges referenced by commits.
• Explicitly close PRs merged into non-default branches when GitHub does not auto-close them.
• Validate PR numbers, yes/no answers, and backport branch names before proceeding.
• Derive lead and co-authors from GitHub-linked commits, defaulting the lead author to the PR author.
• Apply Black formatting to the merge script.

How was this patch tested?

• python3 -m py_compile dev/merge_kyuubi_pr.py
• python3 -m doctest dev/merge_kyuubi_pr.py
• black --check dev/merge_kyuubi_pr.py
• git diff --check

Was this patch assisted by generative AI tooling?

Assisted-by: Codex:GLM 5.3