refactor: move reject_futures to EventQueue, fix test latency and doctest

- Move future rejection logic from AsyncEngine into EventQueue.reject_futures()
  to respect encapsulation (avoids reaching into PriorityQueue internals)
- Reduce asyncio.sleep in test_issue509 from 0.1s to 0.01s
- Convert plain python block in docs/async.md to a testable doctest

Author: fgmacedo

docs/async.md

@@ -193,11 +193,16 @@ compound states, parallel states, history pseudo-states, eventless transitions,
 and `done.state` events — are fully supported in async code. The same
 `activate_initial_state()` pattern applies:
 
-```python
-async def run():
-    sm = MyStateChart()
-    await sm.activate_initial_state()
-    await sm.send("event")
+```py
+>>> async def run():
+...     sm = AsyncStateMachine()
+...     await sm.activate_initial_state()
+...     result = await sm.send("advance")
+...     return result
+
+>>> asyncio.run(run())
+42
+
 ```
 
 ### Concurrent event sending

statemachine/engines/async_.py

@@ -70,14 +70,8 @@ def _reject_future(future: "asyncio.Future[object] | None", exc: Exception):
             future.set_exception(exc)
 
     def _reject_pending_futures(self, exc: Exception):
-        """Reject all unresolved futures in the external queue.
-
-        Called when the processing loop exits abnormally so that coroutines
-        awaiting their futures don't hang forever.
-        """
-        with self.external_queue.queue.mutex:
-            for trigger_data in self.external_queue.queue.queue:
-                self._reject_future(trigger_data.future, exc)
+        """Reject all unresolved futures in the external queue."""
+        self.external_queue.reject_futures(exc)
 
     # --- Callback dispatch overrides (async versions of BaseEngine methods) ---
 

statemachine/engines/base.py

@@ -59,6 +59,18 @@ def clear(self):
         with self.queue.mutex:
             self.queue.queue.clear()
 
+    def reject_futures(self, exc: Exception):
+        """Reject all unresolved futures in the queue.
+
+        Called when the processing loop exits abnormally so that coroutines
+        awaiting their futures don't hang forever.
+        """
+        with self.queue.mutex:
+            for trigger_data in self.queue.queue:
+                future = trigger_data.future
+                if future is not None and not future.done():
+                    future.set_exception(exc)
+
     def remove(self, send_id: str):
         # We use the internal `queue` to make thins faster as the mutex
         # is protecting the block below

tests/testcases/test_issue509.py

@@ -31,7 +31,7 @@ class Issue509SC(StateChart):
     )
 
     async def do_nothing(self, name):
-        await asyncio.sleep(0.1)
+        await asyncio.sleep(0.01)
         return f"Did nothing via {name}"
 
     def raise_exception(self):