Fix observatory checkout failing on commit hash ref

actions/checkout treats short commit hashes as branch names, causing
fetch failures. The checkout only needs the config file from the
repo, so use the default branch instead of a specific ref.

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

.github/workflows/run_observatory_tests.yml

@@ -34,8 +34,6 @@ jobs:
     steps:
       - name: Checkout repository
         uses: actions/checkout@v4
-        with:
-          ref: ${{ inputs.commit_hash || github.sha }}
 
       - name: Validate tag input
         env: