Remove found_descendents param from get_flat_relative_ids (#31559)

By the looks of it, this param is unused.  Since the class is designated private, it is permissable to remove it.

(cherry picked from commit 0cbc0dc)

airflow/models/abstractoperator.py

@@ -154,37 +154,38 @@ def get_direct_relative_ids(self, upstream: bool = False) -> set[str]:
             return self.upstream_task_ids
         return self.downstream_task_ids
 
-    def get_flat_relative_ids(
-        self,
-        upstream: bool = False,
-        found_descendants: set[str] | None = None,
-    ) -> set[str]:
-        """Get a flat set of relative IDs, upstream or downstream."""
+    def get_flat_relative_ids(self, *, upstream: bool = False) -> set[str]:
+        """
+        Get a flat set of relative IDs, upstream or downstream.
+
+        Will recurse each relative found in the direction specified.
+
+        :param upstream: Whether to look for upstream or downstream relatives.
+        """
         dag = self.get_dag()
         if not dag:
             return set()
 
-        if found_descendants is None:
-            found_descendants = set()
+        relatives: set[str] = set()
 
         task_ids_to_trace = self.get_direct_relative_ids(upstream)
         while task_ids_to_trace:
             task_ids_to_trace_next: set[str] = set()
             for task_id in task_ids_to_trace:
-                if task_id in found_descendants:
+                if task_id in relatives:
                     continue
                 task_ids_to_trace_next.update(dag.task_dict[task_id].get_direct_relative_ids(upstream))
-                found_descendants.add(task_id)
+                relatives.add(task_id)
             task_ids_to_trace = task_ids_to_trace_next
 
-        return found_descendants
+        return relatives
 
     def get_flat_relatives(self, upstream: bool = False) -> Collection[Operator]:
         """Get a flat list of relatives, either upstream or downstream."""
         dag = self.get_dag()
         if not dag:
             return set()
-        return [dag.task_dict[task_id] for task_id in self.get_flat_relative_ids(upstream)]
+        return [dag.task_dict[task_id] for task_id in self.get_flat_relative_ids(upstream=upstream)]
 
     def _iter_all_mapped_downstreams(self) -> Iterator[MappedOperator | MappedTaskGroup]:
         """Return mapped nodes that are direct dependencies of the current task.