fix: convert boolean PBC to tensor for System constructor

TorchSim's SimState.pbc is a boolean, but metatomic's System constructor
expects a Tensor. Convert the boolean to a 3-element boolean tensor.

Author: HaoZeke

PR-167-REVIEW-RESPONSES.md

@@ -26,10 +26,20 @@ Move the TorchSim model integration upstream from the TorchSim repository into m
 | 12 | Add support for output variants | ✅ DONE | Added `variants` parameter and `pick_output()` usage in both initialization and energy check |
 | 13 | Extended model outputs API | ⏭️ DEFERRED | Waiting for TorchSim property calculator API to stabilize |
 
+## CI Fixes
+
+### PBC Tensor Conversion (6241b33)
+Fixed TorchSim integration tests failing with:
+```
+RuntimeError: __init__() Expected a value of type 'Tensor' for argument 'pbc' but instead found type 'bool'.
+```
+
+**Fix:** Convert `state.pbc` (boolean) to a 3-element boolean tensor before passing to `System()` constructor.
+
 ## Final Commit
 
 ```
-b82a9c0 feat: add metatomic-torchsim package for TorchSim integration  (Rohit Goswami)
+80e1b9c fix: convert boolean PBC to tensor for System constructor  (Rohit Goswami)
 ```
 
 ### Author Attribution:

python/metatomic_torchsim/metatomic_torchsim/_model.py

@@ -240,12 +240,19 @@ def forward(self, state: "ts.SimState") -> Dict[str, torch.Tensor]:
                 sys_cell = sys_cell @ strain
                 strains.append(strain)
 
+            # Convert boolean PBC to tensor (System expects Tensor, not bool)
+            pbc_tensor = torch.tensor(
+                [state.pbc, state.pbc, state.pbc],
+                device=self._device,
+                dtype=torch.bool,
+            )
+
             systems.append(
                 System(
                     positions=sys_positions,
                     types=sys_types,
                     cell=sys_cell,
-                    pbc=state.pbc,
+                    pbc=pbc_tensor,
                 )
             )