Make sure we return after gracefully shutdown

# Motivation

After calling `shutdownGracefully` in our `ServiceGroup` we know that one of two things happened:
1. We got an error so something went wrong
2. We got no error so all services finished

In the 2. case, there might still be a few child tasks in the task group e.g. the graceful shutdown timeout task. We are currently still iterating the remaining child task results which might lead into unexpected fatal errors.

# Modification
This PR makes sure that after we have successfully shutdown we are stopping iterating the child task results.

# Result
No more unexpected fatal errors.

Author: FranzBusch

Sources/ServiceLifecycle/ServiceGroup.swift

@@ -332,6 +332,7 @@ public actor ServiceGroup: Sendable {
                                 group: &group,
                                 gracefulShutdownManagers: gracefulShutdownManagers
                             )
+                            return .success(())
                         } catch {
                             return .failure(error)
                         }
@@ -425,6 +426,7 @@ public actor ServiceGroup: Sendable {
                                 group: &group,
                                 gracefulShutdownManagers: gracefulShutdownManagers
                             )
+                            return .success(())
                         } catch {
                             return .failure(error)
                         }
@@ -451,6 +453,7 @@ public actor ServiceGroup: Sendable {
                             group: &group,
                             gracefulShutdownManagers: gracefulShutdownManagers
                         )
+                        return .success(())
                     } catch {
                         return .failure(error)
                     }

Tests/ServiceLifecycleTests/ServiceGroupTests.swift

@@ -1112,6 +1112,33 @@ final class ServiceGroupTests: XCTestCase {
         }
     }
 
+    func testGracefulShutdownWithMaximumDuration() async throws {
+        let mockService = MockService(description: "Service1")
+        let serviceGroup = self.makeServiceGroup(
+            services: [.init(service: mockService)],
+            gracefulShutdownSignals: [.sigalrm],
+            maximumGracefulShutdownDuration: .seconds(0.1)
+        )
+
+        try await withThrowingTaskGroup(of: Void.self) { group in
+            group.addTask {
+                try await serviceGroup.run()
+            }
+
+            var eventIterator = mockService.events.makeAsyncIterator()
+            await XCTAsyncAssertEqual(await eventIterator.next(), .run)
+
+            await serviceGroup.triggerGracefulShutdown()
+
+            await XCTAsyncAssertEqual(await eventIterator.next(), .shutdownGracefully)
+
+
+            await mockService.resumeRunContinuation(with: .success(()))
+
+            try await XCTAsyncAssertNoThrow(await group.next())
+        }
+    }
+
     func testGracefulShutdownEscalation_whenNoCancellationEscalation() async throws {
         let mockService = MockService(description: "Service1")
         let serviceGroup = self.makeServiceGroup(