[Staking Async] Saturating accrue era reward points (#9186)

Replaces regular addition with saturating addition when accumulating era
reward points in `pallet-staking-async` to prevent potential overflow.

---------

Co-authored-by: Bastian Köcher <[email protected]>

Author: 2 people

prdoc/pr_9186.prdoc

@@ -0,0 +1,9 @@
+title: 'pallet-staking-async: Use saturating addition for era reward points'
+doc:
+- audience: Runtime Dev
+  description: |-
+    This PR replaces regular addition with saturating addition when accumulating era reward points in
+    pallet-staking-async to prevent potential overflow.
+crates:
+- name: pallet-staking-async
+  bump: patch

substrate/frame/staking-async/src/session_rotation.rs

@@ -364,7 +364,7 @@ impl<T: Config> Eras<T> {
 								era_rewards.individual.try_insert(validator, points).defensive();
 						},
 					}
-					era_rewards.total += points;
+					era_rewards.total.saturating_accrue(points);
 				}
 			});
 		}