Improve PR number handling in workflow (microsoft#3302)

* Improve PR number handling in workflow

Refine PR number extraction and validation method.

* Update .github/workflows/python-test-coverage-report.yml

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Fix error message for invalid PR number

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: markwallace-microsoft

.github/workflows/python-test-coverage-report.yml

@@ -34,9 +34,16 @@ jobs:
         # because the workflow_run event does not have access to the PR number
         # The PR number is needed to post the comment on the PR
         run: |
-          PR_NUMBER=$(cat pr_number)
-          echo "PR number: $PR_NUMBER"
-          echo "PR_NUMBER=$PR_NUMBER" >> $GITHUB_ENV
+          if [ ! -s pr_number ]; then
+            echo "PR number file 'pr_number' is missing or empty"
+            exit 1
+          fi
+          PR_NUMBER=$(head -1 pr_number | tr -dc '0-9')
+          if [ -z "$PR_NUMBER" ]; then
+            echo "PR number file 'pr_number' does not contain a valid PR number"
+            exit 1
+          fi
+          echo "PR_NUMBER=$PR_NUMBER" >> "$GITHUB_ENV"
       - name: Pytest coverage comment
         id: coverageComment
         uses: MishaKav/pytest-coverage-comment@v1.2.0