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/dotnet-verify-samples.yml

@@ -41,7 +41,7 @@ jobs:
     environment: 'integration'
     timeout-minutes: 90
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
         with:
           persist-credentials: false
           sparse-checkout: |
@@ -52,13 +52,13 @@ jobs:
             declarative-agents
 
       - name: Setup dotnet
-        uses: actions/setup-dotnet@v5.2.0
+        uses: actions/setup-dotnet@c2fa09f4bde5ebb9d1777cf28262a3eb3db3ced7 # v5.2.0
         with:
           global-json-file: ${{ github.workspace }}/dotnet/global.json
 
       - name: Azure CLI Login
         if: github.event_name != 'pull_request'
-        uses: azure/login@v2
+        uses: azure/login@a457da9ea143d694b1b9c7c869ebb04ebe844ef5 # v2
         with:
           client-id: ${{ secrets.AZURE_CLIENT_ID }}
           tenant-id: ${{ secrets.AZURE_TENANT_ID }}
@@ -123,7 +123,7 @@ jobs:
 
       - name: Upload results
         if: always()
-        uses: actions/upload-artifact@v7
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7
         with:
           name: verify-samples-results
           path: |