DX-1573: raise error on parallel execution with asyncio.gather() (#24)

* feat: raise error on parallel execution with asyncio.gather()

* refactor: remove unnecessary lock

upstash_workflow/asyncio/context/auto_executor.py

@@ -28,6 +28,7 @@ def __init__(self, context: AsyncWorkflowContext[Any], steps: List[DefaultStep])
         self.step_count: int = 0
         self.plan_step_count: int = 0
         self.executing_step: Union[str, Literal[False]] = False
+        self._already_executed: bool = False
 
     async def add_step(self, step_info: _BaseLazyStep[TResult]) -> TResult:
         self.step_count += 1
@@ -48,6 +49,12 @@ async def run_single(self, lazy_step: _BaseLazyStep[TResult]) -> Any:
             _validate_step(lazy_step, step)
             return step.out
 
+        if self._already_executed:
+            raise WorkflowError(
+                "Running parallel steps is not yet available in workflow-py. Ensure that you are awaiting the steps sequentially."
+            )
+        self._already_executed = True
+
         result_step = await lazy_step.get_result_step(NO_CONCURRENCY, self.step_count)
         await self.submit_steps_to_qstash([result_step], [lazy_step])
         return result_step.out