fix: add checkout step for test-reporter and pin to commit SHA

- Add checkout step before downloading artifacts in test-report job
  This fixes 'fatal: not a git repository' error from dorny/test-reporter
- Pin dorny/test-reporter to v1.9.1 commit SHA for security

The test-reporter action requires git to create check runs properly.

Author: RossBugginsNHS

.github/workflows/stage-2-test.yaml

@@ -119,22 +119,25 @@ jobs:
     permissions:
       checks: write
     steps:
+      - name: "Checkout code"
+        uses: actions/checkout@v5
+
       - name: "Download test results"
         uses: actions/download-artifact@v5
         with:
           name: unit-tests
           path: test-results
 
       - name: "Publish Jest test report"
-        uses: dorny/test-reporter@v1
+        uses: dorny/test-reporter@31a54ee7ebcacc03a09ea97a7e5465a47b84aea5 # v1.9.1
         with:
           name: Jest Test Report
           path: 'test-results/**/junit.xml'
           reporter: jest-junit
           fail-on-error: false
 
       - name: "Publish pytest test report"
-        uses: dorny/test-reporter@v1
+        uses: dorny/test-reporter@31a54ee7ebcacc03a09ea97a7e5465a47b84aea5 # v1.9.1
         with:
           name: Pytest Test Report
           path: 'test-results/**/pytest-report.xml'