ci: require two approvers for release-please PRs (#1144)

Safety net pipeline to protect from accidental release creation which
recognizes `release-please` PRs based on the `a2a-bot` author and
`release-please--` branch name prefix and requires **2 approvers** with
at least `write` level access. This is **an addition** on top of
built-in GitHub write access check. Release PRs could never be merged by
non-maintainers, but now the requirement is 2 maintainers.

Published via a commit status instead of a workflow failure to avoid
"spam" emails of workflow failures, this check will be added as
mandatory. For non-release PRs passes immediately.

Tested on fork:
1. "release" PR: ishymko#7
<img width="784" height="94" alt="image"
src="https://github.com/user-attachments/assets/8fa922d5-cd04-46ca-a0ca-1de099fd7a46"
/>

2. "normal" PR: ishymko#6
<img width="678" height="102" alt="image"
src="https://github.com/user-attachments/assets/e7ef3242-9aa6-454c-bf53-d6d1acddec1f"
/>

Author: ishymko

.github/workflows/release-pr-approvals.yml

@@ -0,0 +1,76 @@
+name: release-pr-approvals
+
+# Requires N approvals from users with write access before a release-please PR
+# can be merged. Posts a commit status (context "release-pr-approvals") so it can
+# be added as a required status check on the protected branch; the job itself
+# stays green. Normal PRs pass immediately.
+#
+# A PR counts as a release PR based on the branch name and author.
+on:
+  pull_request_target:
+    types: [opened, reopened, synchronize]
+  pull_request_review:
+    types: [submitted, dismissed, edited]
+
+permissions:
+  contents: read
+  pull-requests: read
+  statuses: write
+
+concurrency:
+  group: release-pr-approvals-${{ github.event.pull_request.number }}
+  cancel-in-progress: true
+
+jobs:
+  approvals:
+    runs-on: ubuntu-latest
+    env:
+      REQUIRED_APPROVALS: "2"
+      RELEASE_BRANCH_PREFIX: "release-please--"
+      RELEASE_AUTHOR: "a2a-bot"
+    steps:
+      - uses: actions/github-script@3a2844b7e9c422d3c10d287c895573f7108da1b3 # v9.0.0
+        with:
+          script: |
+            const CONTEXT = 'release-pr-approvals';
+            const required = Number(process.env.REQUIRED_APPROVALS);
+            const pr = context.payload.pull_request;
+            const { owner, repo } = context.repo;
+
+            const setStatus = (state, description) =>
+              github.rest.repos.createCommitStatus({
+                owner, repo, sha: pr.head.sha, context: CONTEXT, state, description,
+              });
+
+            const isRelease =
+              pr.head.ref.startsWith(process.env.RELEASE_BRANCH_PREFIX) &&
+              pr.user?.login === process.env.RELEASE_AUTHOR;
+
+            if (!isRelease) {
+              await setStatus('success', 'Not a release PR');
+              return;
+            }
+
+            const reviews = await github.paginate(github.rest.pulls.listReviews, {
+              owner, repo, pull_number: pr.number, per_page: 100,
+            });
+
+            // Latest decisive review per user; COMMENTED does not change approval state.
+            const latest = new Map();
+            for (const r of reviews) {
+              if (!r.user || r.state === 'COMMENTED') continue;
+              latest.set(r.user.login, r.state);
+            }
+
+            let approvals = 0;
+            for (const [login, state] of latest) {
+              if (state !== 'APPROVED') continue;
+              const { data: perm } = await github.rest.repos.getCollaboratorPermissionLevel({
+                owner, repo, username: login,
+              });
+              if (perm.permission === 'admin' || perm.permission === 'write') approvals++;
+            }
+
+            const description = `${approvals}/${required} approvals`;
+            await setStatus(approvals >= required ? 'success' : 'failure', description);
+            core.info(description);