CI fix: skip plot-source test on isolated-install CI

`test_event_study_plot_uses_stored_pointwise_ci_endpoints` imported
`nbformat` and read the notebook from the repo's `docs/tutorials/`
directory. CI Python Tests run from `/tmp/tests/` (isolated install
of the wheel, no repo-tree access) and don't include nbformat in
the runtime deps, so the test errored:

    ModuleNotFoundError: No module named 'nbformat'
    tests/test_t22_had_survey_design_drift.py:409

Two guards added (per `feedback_golden_file_pytest_skip` — same
pattern that benchmarks/data/*.json drift tests use):

  1. `nbformat = pytest.importorskip("nbformat")` — skips when
     the optional dep is missing.
  2. `if not nb_path.exists(): pytest.skip(...)` — skips on the
     isolated-install matrix where docs/ isn't copied alongside
     tests/.

The test runs in any environment that has both nbformat and the
repo tree (dev workspace + tutorial-exec CI workflows), which is
where it actually adds value. The Python Tests matrix doesn't
need to lock notebook source against the prose/code mismatch the
test was added to prevent.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: igerber

tests/test_t22_had_survey_design_drift.py

@@ -404,13 +404,18 @@ def test_event_study_plot_uses_stored_pointwise_ci_endpoints():
 
     This is a static check on the notebook source — the plot cell
     runs but produces no return value we can introspect, so we lock
-    the construction at the source level."""
+    the construction at the source level. Skipped on isolated-install
+    CI jobs where ``docs/`` is not copied alongside ``tests/`` and
+    ``nbformat`` is not in the runtime deps (per
+    ``feedback_golden_file_pytest_skip``)."""
     from pathlib import Path
-    import nbformat
+    nbformat = pytest.importorskip("nbformat")
 
     nb_path = (
         Path(__file__).resolve().parents[1] / "docs" / "tutorials" / "22_had_survey_design.ipynb"
     )
+    if not nb_path.exists():
+        pytest.skip(f"Notebook not present at {nb_path} (isolated-install CI)")
     nb = nbformat.read(nb_path, as_version=4)
     plot_cell_src = None
     for cell in nb.cells: