feat: prevent versioned 3P GitHub actions in PR builds (#262)

Add validation step to require commit SHAs instead of version tags for
third-party GitHub actions in workflow files. Repo config `Require
actions to be pinned to a full-length commit SHA` will protect against
this if we missed any others.

### Testing done
* See:
aws-observability/aws-otel-python-instrumentation#475

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choice.

Author: thpierce

.github/workflows/pr-build.yml

@@ -16,14 +16,15 @@ permissions:
   id-token: write
 
 jobs:
-  changelog-check:
+  static-code-checks:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 #v5.0.0
         with:
           fetch-depth: 0
 
       - name: Check CHANGELOG
+        if: always()
         run: |
           # Check if PR is from workflows bot or dependabot
           if [[ "${{ github.event.pull_request.user.login }}" == "aws-application-signals-bot" ]]; then
@@ -51,6 +52,24 @@ jobs:
           
           echo "It looks like you didn't add an entry to CHANGELOG.md. If this change affects the SDK behavior, please update CHANGELOG.md and link this PR in your entry. If this PR does not need a CHANGELOG entry, you can add the 'Skip Changelog' label to this PR."
           exit 1
+          
+      - name: Check for versioned GitHub actions
+        if: always()
+        run: |
+          # Get changed GitHub workflow/action files
+          CHANGED_FILES=$(git diff --name-only origin/${{ github.base_ref }}..HEAD | grep -E "^\.github/(workflows|actions)/.*\.ya?ml$" || true)
+          
+          if [ -n "$CHANGED_FILES" ]; then
+            # Check for any versioned actions, excluding comments and this validation script
+            VIOLATIONS=$(grep -Hn "uses:.*@v" $CHANGED_FILES | grep -v "grep.*uses:.*@v" | grep -v "#.*@v" || true)
+            if [ -n "$VIOLATIONS" ]; then
+              echo "Found versioned GitHub actions. Use commit SHAs instead:"
+              echo "$VIOLATIONS"
+              exit 1
+            fi
+          fi
+          
+          echo "No versioned actions found in changed files"
 
   build:
     runs-on: ${{ matrix.os }}