add workflow/tests for checking notebook links

.github/workflows/check-binder-files.yml

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+name: Test Binder Notebook Links
+
+on:
+  push:
+    branches: [ main ]
+    paths:
+      - '.binder/files_to_fetch.txt'
+  pull_request:
+    types: [ opened, synchronize, reopened ]
+    branches: [ main ]
+  schedule:
+    # run every Monday at 08:00 UTC to catch broken links automatically
+    - cron: '0 8 * * 1'
+  workflow_dispatch: # allow manual triggering from the Actions tab
+
+jobs:
+  test-notebook-links:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.12'
+
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          pip install pytest
+
+      - name: Run notebook URL unit tests
+        run: |
+          pytest tests/test_binder_files.py -v

.gitignore

@@ -0,0 +1 @@
+*.pyc

tests/test_binder_files.py

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+from pathlib import Path
+import urllib.request
+import urllib.error
+import pytest
+
+REPO_OWNER = "Parcels-code"
+REPO_NAME = "virtualship"
+BRANCH = "main"
+
+# path to the list file relative to the repository root
+LIST_FILE = Path(__file__).parent.parent / ".binder" / "files_to_fetch.txt"
+
+
+def get_target_files():
+    """Reads non-empty, non-comment lines from files_to_fetch.txt."""
+    if not LIST_FILE.exists():
+        pytest.fail(f"List file not found at: {LIST_FILE}")
+
+    files = []
+    with open(LIST_FILE, "r", encoding="utf-8") as f:
+        for line in f:
+            line = line.strip()
+            if line and not line.startswith("#"):
+                files.append(line)
+    return files
+
+
+@pytest.mark.parametrize("file_path", get_target_files())
+def test_remote_notebook_exists(file_path):
+    """Sends a HEAD request to verify each notebook URL is active (HTTP 200)."""
+    raw_url = f"https://raw.githubusercontent.com/{REPO_OWNER}/{REPO_NAME}/{BRANCH}/{file_path}"
+
+    req = urllib.request.Request(raw_url, method="HEAD")
+
+    try:
+        with urllib.request.urlopen(req) as response:
+            assert response.status == 200, (
+                f"Expected status 200, got {response.status} for {raw_url}"
+            )
+    except urllib.error.HTTPError as e:
+        pytest.fail(
+            f"Failed to reach notebook at '{file_path}'.\n"
+            f"URL: {raw_url}\n"
+            f"HTTP Error: {e.code} {e.reason}\n"
+            f"Please update '.binder/files_to_fetch.txt' or report on the issue tracker: "
+            f"https://github.com/{REPO_OWNER}/{REPO_NAME}/issues"
+        )
+    except urllib.error.URLError as e:
+        pytest.fail(f"Network error while connecting to {raw_url}: {e.reason}")