ci: pin third-party GitHub Actions to commit SHAs (#5972)

Replaces every floating tag in our workflow and composite action files
with an immutable 40-character commit SHA, keeping the original `# vX`
comment so Dependabot can still propose version bumps. 186 occurrences
across 25 workflows and 2 composite actions.

Also widens the github-actions Dependabot entry to use the plural
`directories` key with `/.github/actions/*` so composite actions under
`.github/actions/<name>/action.yml` are kept up to date. Previously
Dependabot only scanned `.github/workflows` and the repo-root
`action.yml`, leaving our `python-setup` and `sample-validation-setup`
composite actions unmaintained.

.github/workflows/python-tests.yml

@@ -27,7 +27,7 @@ jobs:
       run:
         working-directory: python
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
       - name: Set up python and install the project
         id: python-setup
         uses: ./.github/actions/python-setup
@@ -46,7 +46,7 @@ jobs:
       # Surface failing tests
       - name: Surface failing tests
         if: always()
-        uses: pmeier/pytest-results-action@v0.7.2
+        uses: pmeier/pytest-results-action@20b595761ba9bf89e115e875f8bc863f913bc8ad # v0.7.2
         with:
           path: ./python/pytest.xml
           summary: true