fix: skip the jax-only cluster simulator and tutorial when jax is absent

CI's Python 3.13 leg deliberately runs the lean no-optional-deps path
without jax; the new cluster simulator imports jax for its jitted
multi-plane PointSolver, failing that leg (and the cluster tutorial with
it via the auto-simulate subprocess). Both now use the established
importlib find_spec / sys.exit(0) guard — the same idiom as the two
jax-only chapter 3 tutorials — exiting cleanly with an explanatory
message when jax is missing, verified both with jax removed (clean skip)
and present (full run). Notebooks regenerated.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BxKfSZisjnEn91LRGkN4SU

Author: claude

notebooks/chapter_4_scaling_up_lensing/tutorial_5_cluster_scale.ipynb

@@ -132,6 +132,17 @@
    "cell_type": "code",
    "metadata": {},
    "source": [
+    "\n",
+    "import importlib.util\n",
+    "import sys\n",
+    "\n",
+    "if importlib.util.find_spec(\"jax\") is None:\n",
+    "    print(\n",
+    "        \"Skipping this tutorial: it requires the `jax` package (used to \"\n",
+    "        \"accelerate the multi-plane point-source solves), which is not \"\n",
+    "        \"installed (install with `pip install autolens[optional]`).\"\n",
+    "    )\n",
+    "    sys.exit(0)\n",
     "\n",
     "from autolens import jax_wrapper  # Sets JAX environment before other imports\n",
     "\n",

notebooks/simulator/cluster.ipynb

@@ -95,6 +95,16 @@
    "cell_type": "code",
    "metadata": {},
    "source": [
+    "\n",
+    "import importlib.util\n",
+    "import sys\n",
+    "\n",
+    "if importlib.util.find_spec(\"jax\") is None:\n",
+    "    print(\n",
+    "        \"Skipping this simulator: it requires the `jax` package, which is not \"\n",
+    "        \"installed (install with `pip install autolens[optional]`).\"\n",
+    "    )\n",
+    "    sys.exit(0)\n",
     "\n",
     "from autolens import jax_wrapper  # Sets JAX environment before other imports\n",
     "\n",

scripts/chapter_4_scaling_up_lensing/tutorial_5_cluster_scale.py

@@ -85,6 +85,17 @@
 redshift for every source.
 """
 
+import importlib.util
+import sys
+
+if importlib.util.find_spec("jax") is None:
+    print(
+        "Skipping this tutorial: it requires the `jax` package (used to "
+        "accelerate the multi-plane point-source solves), which is not "
+        "installed (install with `pip install autolens[optional]`)."
+    )
+    sys.exit(0)
+
 from autolens import jax_wrapper  # Sets JAX environment before other imports
 
 # from autolens import setup_notebook; setup_notebook()

scripts/simulator/cluster.py

@@ -48,6 +48,16 @@
 If any code in this script is unclear, refer to the `autolens_workspace/*/cluster/simulator.ipynb` notebook.
 """
 
+import importlib.util
+import sys
+
+if importlib.util.find_spec("jax") is None:
+    print(
+        "Skipping this simulator: it requires the `jax` package, which is not "
+        "installed (install with `pip install autolens[optional]`)."
+    )
+    sys.exit(0)
+
 from autolens import jax_wrapper  # Sets JAX environment before other imports
 
 # from autolens import setup_notebook; setup_notebook()