Fix Rust backend test for loocv_grid_search 7-value return

Update test to unpack 7 values from loocv_grid_search (added first_failed_obs
in round 8) and add assertion for the new return value.

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

Author: igerber

tests/test_rust_backend.py

@@ -970,7 +970,7 @@ def test_loocv_grid_search_returns_valid_params(self):
         lambda_unit = np.array([0.0, 1.0], dtype=np.float64)
         lambda_nn = np.array([0.0, 0.1], dtype=np.float64)
 
-        best_lt, best_lu, best_ln, score, n_valid, n_attempted = loocv_grid_search(
+        best_lt, best_lu, best_ln, score, n_valid, n_attempted, first_failed = loocv_grid_search(
             Y, D, control_mask, time_dist,
             lambda_time, lambda_unit, lambda_nn,
             50, 100, 1e-6, 42
@@ -985,6 +985,8 @@ def test_loocv_grid_search_returns_valid_params(self):
         assert n_valid >= 0
         assert n_attempted >= 0
         assert n_valid <= n_attempted
+        # Check first_failed is None or a valid (unit, time) tuple
+        assert first_failed is None or (isinstance(first_failed, tuple) and len(first_failed) == 2)
 
     def test_bootstrap_variance_shape(self):
         """Test bootstrap returns correct shapes."""