Fix doc-snippet test failures from PR 2.5 plot_bacon insertion

CI's "Validate RST code snippets" job failed with 2 errors after the
plot_bacon section was inserted in docs/api/visualization.rst:

1. block8 (new plot_bacon example) used wrong API:
   BaconDecomposition.fit() takes first_treat=, not treatment=.
   Fixed to first_treat='first_treat'.

2. block9 (Plotly Backend, was block8 before insertion) raised NameError
   on 'results' because it relies on context from earlier blocks. The
   _CONTEXT_DEPENDENT_SNIPPETS allowlist had api_visualization:block8
   which now points to my new plot_bacon block (which is standalone and
   does NOT need leniency); the actual context-dependent block moved to
   index 9. Updated allowlist entry to api_visualization:block9.

Verified locally: both blocks now pass under
PYTHONPATH=. DIFF_DIFF_BACKEND=python pytest

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

Author: igerber

docs/api/visualization.rst

@@ -204,7 +204,7 @@ Example
 
    bd = BaconDecomposition()
    results = bd.fit(data, outcome='y', unit='unit_id',
-                    time='period', treatment='treated')
+                    time='period', first_treat='first_treat')
 
    # Scatter of 2x2 comparisons by weight, colored by comparison type
    ax = plot_bacon(results)

tests/test_doc_snippets.py

@@ -358,7 +358,7 @@ def _restore_datasets_module():
     "api_bacon:block2",
     "api_visualization:block2",
     "api_visualization:block3",
-    "api_visualization:block8",
+    "api_visualization:block9",
     "python_comparison:block5",
     "quickstart:block3",
     "quickstart:block9",