[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

[wangzhigang1999]

The referenced-event fallback does not recognize merge messages generated by this script

_MERGE_FOOTER_RE requires Authored-by or Lead-authored-by to appear immediately after the Closes #<pr> from ... paragraph. However, merge_pr inserts the linked issue and original commit summaries between those paragraphs.

This does not affect the first merge of a new PR or backports completed in the same script run. It affects a later invocation that uses an already-merged PR number to continue backporting.

I reproduced this with the existing release-branch PR [#7116](#7116), which was merged into branch-1.10 as commit [7c6d5658](7c6d565). Its relevant events are:

referenced  7c6d5658298447ba440cd424d232b4d062573a69
closed      commit_id=None

The merge message contains:

Closes #7116 from turboFei/fix_conflicts.

Closes #7101

f25f487e8 [Wang, Fei] [KYUUBI #7101][1.10][FOLLOWUP] Fix code broken

Authored-by: Wang, Fei <fwang12@ebay.com>

Running the code from this PR against those events gives:

find_merge_commit(7116, events)
# (None, None)

Since the closed event has no commit ID, find_merge_commit must use the referenced commit. The regex rejects its message, so a later invocation fails to recognize that PR #7116 has already landed and continues through the normal merge path instead of the backport path.