ci: dependabot version updates + auto-merge parity

Brings pgraph-python in line with RTB/MVTB/bdsim: a dependabot.yml
(github-actions ecosystem), an auto-merge workflow for patch/minor
bumps, and an all-tests-passed gate job branch protection can require.

Author: petercorke

.github/dependabot.yml

@@ -0,0 +1,8 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "monthly"
+    labels:
+      - "dependencies"

.github/workflows/ci.yml

@@ -48,6 +48,19 @@ jobs:
         continue-on-error: true
         run: mypy src/pgraph/PGraph.py
 
+  all-tests-passed:
+    name: All tests passed
+    needs: unittest
+    runs-on: ubuntu-latest
+    if: always()
+    steps:
+      - name: Check test matrix result
+        run: |
+          if [ "${{ needs.unittest.result }}" != "success" ]; then
+            echo "test matrix did not succeed: ${{ needs.unittest.result }}"
+            exit 1
+          fi
+
   codecov:
     needs: unittest
     runs-on: ubuntu-latest

.github/workflows/dependabot-auto-merge.yml

@@ -0,0 +1,25 @@
+name: Dependabot auto-merge
+
+on: pull_request
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  dependabot:
+    runs-on: ubuntu-latest
+    if: ${{ github.actor == 'dependabot[bot]' }}
+    steps:
+      - name: Dependabot metadata
+        id: metadata
+        uses: dependabot/fetch-metadata@v3
+        with:
+          github-token: "${{ secrets.GITHUB_TOKEN }}"
+
+      - name: Enable auto-merge for patch/minor bumps
+        if: ${{ steps.metadata.outputs.update-type == 'version-update:semver-patch' || steps.metadata.outputs.update-type == 'version-update:semver-minor' }}
+        run: gh pr merge --auto --merge "$PR_URL"
+        env:
+          PR_URL: ${{ github.event.pull_request.html_url }}
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}