Fix global bootstrap NaN SE test to disable Rust backend

The test patches _fit_joint_with_fixed_lambda to force failures, but
on CI with Rust available, the Rust bootstrap path runs instead of the
Python fallback. Disable Rust backend in the test to exercise the
Python return path.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: igerber

tests/test_trop.py

@@ -3583,6 +3583,7 @@ class TestTROPBootstrapNaNSE:
     def test_global_bootstrap_zero_draws_returns_nan_se(self):
         """Global bootstrap with 0 successful draws returns NaN SE, not 0.0."""
         from unittest.mock import patch
+        import sys
 
         df = TestTROPNValidTreated._make_panel()
 
@@ -3595,8 +3596,12 @@ def test_global_bootstrap_zero_draws_returns_nan_se(self):
             seed=42,
         )
 
-        # Patch _fit_joint_with_fixed_lambda to always raise (all bootstrap iters fail)
-        with patch.object(TROP, '_fit_joint_with_fixed_lambda',
+        # Disable Rust backend so Python fallback path is tested,
+        # then patch _fit_joint_with_fixed_lambda to always raise
+        trop_module = sys.modules['diff_diff.trop']
+        with patch.object(trop_module, 'HAS_RUST_BACKEND', False), \
+             patch.object(trop_module, '_rust_bootstrap_trop_variance_joint', None), \
+             patch.object(TROP, '_fit_joint_with_fixed_lambda',
                           side_effect=ValueError("forced failure")):
             with warnings.catch_warnings():
                 warnings.simplefilter("ignore")