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-dependency-range-validation.yml

@@ -22,7 +22,7 @@ jobs:
       UV_PYTHON: "3.13"
       GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
         with:
           fetch-depth: 0
 
@@ -44,7 +44,7 @@ jobs:
       - name: Upload dependency range report
         # Always publish the report so failures are inspectable even when validation fails.
         if: always()
-        uses: actions/upload-artifact@v7
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7
         with:
           name: dependency-range-results
           path: python/scripts/dependencies/dependency-range-results.json
@@ -53,7 +53,7 @@ jobs:
       - name: Create issues for failed dependency candidates
         # Always process the report so failed candidates create actionable tracking issues.
         if: always()
-        uses: actions/github-script@v8
+        uses: actions/github-script@ed597411d8f924073f98dfc5c65a23a2325f34cd # v8
         with:
           script: |
             const fs = require("fs")