Fix Ubuntu CI build by replacing autoreleasepool with do blocks

Replace autoreleasepool blocks with do blocks in SubgraphCompatibilityTests
  to fix build failures on Ubuntu/Linux where autoreleasepool is not available
  (Darwin-only feature). The do blocks maintain the same scoping behavior
  needed for the tests while being cross-platform compatible.

Author: Kyle-Ye

Tests/OpenAttributeGraphCompatibilityTests/Graph/SubgraphCompatibilityTests.swift

@@ -34,53 +34,46 @@ struct SubgraphCompatibilityTests {
         @Test
         func observerNotifiedOnSubgraphDestroyed() {
             var notifiedCount = 0
-            autoreleasepool {
+            do {
                 let graph = Graph()
-
-                autoreleasepool {
+                do {
                     let subgraph = Subgraph(graph: graph)
                     let _ = subgraph.addObserver {
                         notifiedCount += 1
                     }
                 }
-
                 #expect(notifiedCount == 1)
             }
-
             // Observers aren't notified more than once
             #expect(notifiedCount == 1)
         }
 
         @Test
         func observerNotifiedOnGraphDestroyed() {
             var notifiedCount = 0
-            autoreleasepool {
+            do {
                 let graph = Graph()
                 let subgraph = Subgraph(graph: graph)
                 let _ = subgraph.addObserver {
                     notifiedCount += 1
                 }
-
                 #expect(notifiedCount == 0)
             }
-
             #expect(notifiedCount == 1)
         }
 
         @Test
         func removedObserverNotNotified() {
             var notifiedCount = 0
-            autoreleasepool {
+            do {
                 let graph = Graph()
                 let subgraph = Subgraph(graph: graph)
                 let observerID = subgraph.addObserver {
                     notifiedCount += 1
                 }
                 subgraph.removeObserver(observerID)
             }
-
             #expect(notifiedCount == 0)
         }
-
     }
 }