ci: checkout the correct commit SHA for workflow_run trigger

Use workflow_run.head_sha to check out the PR/push commit that
triggered the Build and Test workflow, rather than the default branch.
Falls back to github.sha for manual dispatch.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jmesnil

.github/workflows/run-tck.yml

@@ -29,6 +29,8 @@ jobs:
     steps:
       - name: Checkout a2a-java
         uses: actions/checkout@v6
+        with:
+          ref: ${{ github.event.workflow_run.head_sha || github.sha }}
       - name: Set up JDK ${{ matrix.java-version }}
         uses: actions/setup-java@v5
         with: