testing: add test workflow for composite action

.github/workflows/check-blocking-labels.yml

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+# SPDX-FileCopyrightText: Contributors to the Power Grid Model project <powergridmodel@lfenergy.org>
+#
+# SPDX-License-Identifier: MPL-2.0
+
+name: Check Blocking Labels
+
+on:
+  # run pipeline on pull request
+  pull_request:
+    types:
+      - opened
+      - synchronize
+      - labeled
+      - unlabeled
+  # run pipeline on merge queue
+  merge_group:
+  # run this workflow manually from the Actions tab
+  workflow_dispatch:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}-blocking-labels
+  cancel-in-progress: true
+
+jobs:
+  check-blocking-labels:
+    runs-on: ubuntu-latest
+    steps:
+
+      - name: do-not-merge
+        if: contains(github.event.pull_request.labels.*.name, 'do-not-merge')
+        run: |
+          echo "This pull request should not be merged (do-not-merge)"
+          exit 1
+
+      - name: merge-target-first
+        if: contains(github.event.pull_request.labels.*.name, 'merge-target-first')
+        run: |
+          echo "The target branch of this PR should be merged first (merge-target-first)"
+          exit 2
+
+      - name: needs-unit-tests
+        if: contains(github.event.pull_request.labels.*.name, 'needs-unit-tests')
+        run: |
+          echo "This pull request needs (more) unit tests before it may be merged (needs-unit-tests)"
+          exit 3

.github/workflows/test-action.yml

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+# SPDX-FileCopyrightText: Contributors to the Power Grid Model project <powergridmodel@lfenergy.org>
+#
+# SPDX-License-Identifier: MPL-2.0
+
+name: Test the Action
+
+on:
+  # run pipeline on push event of main branch
+  push:
+    branches:
+      - main
+  # run pipeline on pull request
+  pull_request:
+  # run pipeline on merge queue
+  workflow_dispatch:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}-test-action
+  cancel-in-progress: true
+
+jobs:
+  test-pgm-version-bump:
+    runs-on: ubuntu-24.04
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set PyPI Version
+        uses: ./
+
+      - name: Show latest version
+        run: cat PYPI_VERSION

VERSION

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+0.0

VERSION.license

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+SPDX-FileCopyrightText: Contributors to the Power Grid Model project <powergridmodel@lfenergy.org>
+
+SPDX-License-Identifier: MPL-2.0