fix(isaac): mirror real SimEngine abstract surface in fallback ABC

cagataycali's review comment on simulation.py:50 (#47) flagged that the
fallback ABC declared zero @AbstractMethod decorators, so
IsaacSimulation() instantiated cleanly in tests when strands-robots was
absent — masking the abstract-method gap that surfaces the moment the
real ABC is in scope.

Mirror the real strands_robots.simulation.base.SimEngine surface (14
abstract methods) in the fallback. Now `pytest` produces the same
failure mode (25 failed / 33 passed) whether strands-robots is installed
or not, including the dedicated canary
test_isaac_implements_all_abstract_methods. The 4 missing concrete
implementations on IsaacSimulation (list_robots, remove_robot,
remove_object, robot_joint_names) remain a separately tracked gap per
the PR body — this commit just makes the gap visible in CI.

Author: yinsong1986

strands_robots_sim/isaac/simulation.py

@@ -36,10 +36,45 @@
 except (ImportError, ModuleNotFoundError):
     # Fallback: strands-robots < 0.4.0 doesn't have simulation.base yet.
     # Provide a minimal ABC stub so the class can still be defined.
-    from abc import ABC
+    from abc import ABC, abstractmethod
 
     class SimEngine(ABC):  # type: ignore[no-redef]
-        """Minimal fallback ABC when strands-robots.simulation.base is unavailable."""
+        """Minimal fallback ABC when strands-robots.simulation.base is unavailable.
+
+        Mirrors the abstract surface of the real
+        ``strands_robots.simulation.base.SimEngine`` so subclasses fail fast at
+        instantiation if a method is missing -- same contract whether
+        ``strands_robots`` is installed or not.
+        """
+
+        @abstractmethod
+        def create_world(self, **kwargs): ...
+        @abstractmethod
+        def destroy(self): ...
+        @abstractmethod
+        def reset(self): ...
+        @abstractmethod
+        def step(self, n_steps: int = 1): ...
+        @abstractmethod
+        def get_state(self): ...
+        @abstractmethod
+        def add_robot(self, name: str, **kwargs): ...
+        @abstractmethod
+        def remove_robot(self, name: str): ...
+        @abstractmethod
+        def list_robots(self) -> list: ...
+        @abstractmethod
+        def robot_joint_names(self, robot_name: str) -> list: ...
+        @abstractmethod
+        def add_object(self, name: str, **kwargs): ...
+        @abstractmethod
+        def remove_object(self, name: str): ...
+        @abstractmethod
+        def get_observation(self, robot_name=None, *, skip_images=False): ...
+        @abstractmethod
+        def send_action(self, action, robot_name=None, n_substeps: int = 1): ...
+        @abstractmethod
+        def render(self, camera_name: str = "default", width=None, height=None): ...
 
         def cleanup(self) -> None:
             pass