Address PR #110 feedback round 9: fix test fixture mismatch

Update mock return value for _rust_loocv_grid_search to include the
7th value (first_failed_obs) that was added in round 8. The mock was
returning 6 values but the actual Rust function returns 7.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: igerber

tests/test_trop.py

@@ -2081,8 +2081,8 @@ def test_rust_infinite_score_triggers_fallback(self, simple_panel_data):
         )
 
         # Mock Rust function to return infinite score
-        # Return format: (lambda_time, lambda_unit, lambda_nn, score, n_valid, n_attempted)
-        mock_rust_loocv = MagicMock(return_value=(0.5, 0.5, 0.05, np.inf, 0, 100))
+        # Return format: (lambda_time, lambda_unit, lambda_nn, score, n_valid, n_attempted, first_failed_obs)
+        mock_rust_loocv = MagicMock(return_value=(0.5, 0.5, 0.05, np.inf, 0, 100, None))
 
         # Also mock Python LOOCV to return infinity (so Python fallback also fails)
         def always_infinity(*args, **kwargs):