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-integration-tests.yml

@@ -29,7 +29,7 @@ jobs:
     environment: integration
     timeout-minutes: 60
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
         with:
           ref: ${{ inputs.checkout-ref }}
           persist-credentials: false
@@ -50,7 +50,7 @@ jobs:
           echo "COSMOS_EMULATOR_AVAILABLE=true" >> $env:GITHUB_ENV
 
       - 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
 
@@ -63,7 +63,7 @@ jobs:
           done
 
       - name: Azure CLI Login
-        uses: azure/login@v2
+        uses: azure/login@a457da9ea143d694b1b9c7c869ebb04ebe844ef5 # v2
         with:
           client-id: ${{ secrets.AZURE_CLIENT_ID }}
           tenant-id: ${{ secrets.AZURE_TENANT_ID }}