Fix Python linting workflow for fork PRs

The peter-evans/create-pull-request action fails for fork PRs because
it tries to fetch a branch that only exists in the fork. Add conditional
logic to handle fork PRs differently:

- Fork PRs: Run black --check --diff and fail if formatting issues exist
- Same-repo PRs and pushes: Keep existing behavior (run black, create fix PR)

.github/workflows/python-linting.yml

@@ -9,13 +9,26 @@ jobs:
       - uses: actions/checkout@v6
       - uses: actions/setup-python@v6
         with:
-          python-version: '3.10' 
+          python-version: '3.10'
+
+      # Fork PRs: Check formatting and fail with diff if issues exist
+      # (Cannot create fix PR because we don't have write access to fork branches)
+      - name: (Python) Check formatting with black (fork PRs)
+        if: github.event_name == 'pull_request' && github.event.pull_request.head.repo.full_name != github.repository
+        run: |
+          pip install black
+          cd GrpcInterface
+          black --check --diff .
+
+      # Same-repo PRs and pushes: Apply fixes and create PR if needed
       - name: (Python) Use black to do linting
+        if: github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name == github.repository
         run: |
           pip install black
           cd GrpcInterface
           black .
       - uses: peter-evans/create-pull-request@v8
+        if: github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name == github.repository
         with:
           token: ${{ secrets.GITHUB_TOKEN }}
           commit-message: "Python code linting changes detected by black"