ci: Add action to creatae issue on gitflow merge conflicts (#18319)

It's often hard to notice when our Gitflow merge PRs fail due to merge
conflicts and the longer they are open the harder it will be to fix
these merge conflicts eventually.

e.g. #18314

This is an attempt at notifying us better when these happen by creating
an issue in our repo (did not actually test this yet so we might have to
go through some iterations 😅).

Idea by Abhi:
#18317 (review)

Author: andreiborza

.github/workflows/gitflow-merge-conflict.yml

@@ -0,0 +1,107 @@
+name: 'Gitflow: Merge Conflict Issue'
+
+on:
+  pull_request:
+    types: [opened]
+    branches:
+      - develop
+
+jobs:
+  check-merge-conflicts:
+    name: Detect merge conflicts in gitflow PRs
+    runs-on: ubuntu-24.04
+    if: |
+      ${{ contains(github.event.pull_request.labels.*.name, 'Dev: Gitflow') }}
+    permissions:
+      issues: write
+    steps:
+      - name: Check for merge conflicts with retry
+        uses: actions/github-script@v8
+        with:
+          script: |
+            const retryInterval = 30_000;
+            const maxRetries = 10; // (30 seconds * 10 retries) = 5 minutes
+
+            async function isMergeable() {
+              const { data: pr } = await github.rest.pulls.get({
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                pull_number: context.payload.pull_request.number
+              });
+
+              return pr.mergeable;
+            }
+
+            async function sleep(ms) {
+              return new Promise(resolve => setTimeout(resolve, ms));
+            }
+
+            let attempt = 0;
+            let mergeable = null;
+
+            while (attempt < maxRetries) {
+              attempt++;
+              console.log(`Attempt ${attempt}/${maxRetries}: Checking if PR is mergeable...`);
+
+              mergeable = await isMergeable();
+              console.log(`Mergeable: ${mergeable}`);
+
+              // If mergeable is not null, GitHub has finished computing merge state
+              if (mergeable !== null) {
+                break;
+              }
+
+              if (attempt < maxRetries) {
+                console.log(`Waiting ${retryInterval/1000} seconds before retry...`);
+                await sleep(retryInterval);
+              }
+            }
+
+            // Check if we have merge conflicts
+            if (mergeable === false) {
+              const issueTitle = '[Gitflow] Merge Conflict';
+
+              // Check for existing open issues with the same title
+              const { data: existingIssues } = await github.rest.issues.listForRepo({
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                state: 'open',
+                labels: 'Dev: Gitflow'
+              });
+
+              const existingOpenIssue = existingIssues.find(issue =>
+                issue.title === issueTitle && !issue.pull_request
+              );
+
+              if (!existingOpenIssue) {
+                const issueBody = [
+                  '## Gitflow Merge Conflict Detected',
+                  '',
+                  `The automated gitflow PR #${context.payload.pull_request.number} has merge conflicts and cannot be merged automatically.`,
+                  '',
+                  '### How to resolve',
+                  '',
+                  `Follow the steps documented in [docs/gitflow.md](https://github.com/${context.repo.owner}/${context.repo.repo}/blob/develop/docs/gitflow.md#what-to-do-if-there-is-a-merge-conflict):`,
+                  '',
+                  `1. Close the automated PR #${context.payload.pull_request.number}`,
+                  '2. Create a new branch on top of `master` (e.g., `manual-develop-sync`)',
+                  '3. Merge `develop` into this branch with a **merge commit** (fix any merge conflicts)',
+                  '4. Create a PR against `develop` from your branch',
+                  '5. Merge that PR with a **merge commit**'
+                ].join('\n');
+
+                await github.rest.issues.create({
+                  owner: context.repo.owner,
+                  repo: context.repo.repo,
+                  title: issueTitle,
+                  body: issueBody,
+                  labels: ['Dev: Gitflow']
+                });
+
+                console.log('Created new issue for merge conflict');
+              }
+            } else if (mergeable === null) {
+              console.log('Could not determine mergeable state after maximum retries');
+            } else {
+              console.log('No merge conflicts detected - PR can be merged');
+            }