fix(ci): pin Poetry version and update GitHub Actions to prevent install failures

- Pin Poetry to 2.1.1 via pipx (matches poetry.lock) instead of
  curling latest, which broke when Poetry 2.3.4 was released
- Update pre-commit workflow to actions/checkout@v4, setup-python@v5
- Pin pre-commit Python to 3.12 (was unpinned, pulling 3.14 edge)

.github/workflows/pre-commit.yml

@@ -9,6 +9,8 @@ jobs:
   pre-commit:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
-    - uses: actions/setup-python@v3
+    - uses: actions/checkout@v4
+    - uses: actions/setup-python@v5
+      with:
+        python-version: "3.12"
     - uses: pre-commit/action@v3.0.1

.github/workflows/test.yml

@@ -32,7 +32,7 @@ jobs:
       - name: Setup Graphviz
         uses: ts-graphviz/setup-graphviz@v2
       - name: Install poetry
-        run: curl -sSL https://install.python-poetry.org | python3 -
+        run: pipx install poetry==2.1.1
       - name: Run all tests
         run: |
           poetry install