ci: cache .venv instead of ~/.cache/pdm

- Cache virtual environment (includes installed tools)
- Fixes: black/isort/flake8 not found errors
- Cache hit: ~2s install, Cache miss: ~15s install

Author: tracer-mohist

.github/workflows/test.yml

@@ -1,6 +1,6 @@
 # .github/workflows/test.yml
 #
-# CI for pull requests
+# CI for pull requests and push to main
 # Runs test, lint, commit message, and docs checks in parallel
 #
 
@@ -29,13 +29,13 @@ jobs:
         with:
           python-version: '3.12'
 
-      - name: Cache PDM packages
+      - name: Cache virtual environment
         uses: actions/cache@v4
         with:
-          path: ~/.cache/pdm
-          key: ${{ runner.os }}-pdm-${{ hashFiles('pdm.lock') }}
+          path: .venv
+          key: ${{ runner.os }}-venv-${{ hashFiles('pdm.lock') }}
           restore-keys: |
-            ${{ runner.os }}-pdm-
+            ${{ runner.os }}-venv-
 
       - run: pdm install --with dev
       - run: pdm run pytest -v
@@ -52,13 +52,13 @@ jobs:
         with:
           python-version: '3.12'
 
-      - name: Cache PDM packages
+      - name: Cache virtual environment
         uses: actions/cache@v4
         with:
-          path: ~/.cache/pdm
-          key: ${{ runner.os }}-pdm-${{ hashFiles('pdm.lock') }}
+          path: .venv
+          key: ${{ runner.os }}-venv-${{ hashFiles('pdm.lock') }}
           restore-keys: |
-            ${{ runner.os }}-pdm-
+            ${{ runner.os }}-venv-
 
       - run: pdm install --with dev
       - run: pdm run black --check src/ tests/