fix(cua): keep standalone application URLs stable across reuse

packages/experimental/cua-macos/native/Sources/CuaNative/StandaloneApplication.swift

@@ -28,8 +28,8 @@ enum StandaloneApplication {
         let files = FileManager.default
         try files.createDirectory(at: root, withIntermediateDirectories: true, attributes: [.posixPermissions: 0o700])
         let root = root.resolvingSymlinksInPath()
-        let generation = root.appendingPathComponent(runtimeKey)
-        let application = generation.appendingPathComponent(source.lastPathComponent)
+        let generation = root.appendingPathComponent(runtimeKey, isDirectory: true)
+        let application = generation.appendingPathComponent(source.lastPathComponent, isDirectory: true)
         guard !hasApplicationAncestor(application) else {
             throw NativeFailure(code: "NATIVE_APPLICATION_REQUIRED", message: "Computer Use application storage must be outside another .app: \(root.path)")
         }
@@ -50,10 +50,10 @@ enum StandaloneApplication {
             }
             return application
         }
-        let staging = root.appendingPathComponent(".staging-\(UUID().uuidString)")
+        let staging = root.appendingPathComponent(".staging-\(UUID().uuidString)", isDirectory: true)
         try files.createDirectory(at: staging, withIntermediateDirectories: false, attributes: [.posixPermissions: 0o700])
         do {
-            let stagedApplication = staging.appendingPathComponent(source.lastPathComponent)
+            let stagedApplication = staging.appendingPathComponent(source.lastPathComponent, isDirectory: true)
             try files.copyItem(at: source, to: stagedApplication)
             try validate(stagedApplication, runtimeKey: runtimeKey)
             try files.moveItem(at: staging, to: generation)

packages/experimental/cua-macos/native/tests/swift/StandaloneApplicationTests.swift

@@ -111,6 +111,17 @@ func aChangedCachedApplicationIsRejected(file: String) throws {
     }
 }
 
+@Test func aCachedSymlinkCannotRedirectTheHelperIntoAnEnclosingApplication() throws {
+    let f = try ApplicationFixture(); defer { f.close() }
+    let key = try f.key
+    try FileManager.default.createDirectory(at: f.cache, withIntermediateDirectories: true)
+    try FileManager.default.createSymbolicLink(at: f.cache.appendingPathComponent(key),
+                                               withDestinationURL: f.application.deletingLastPathComponent())
+    #expect(throws: (any Error).self) {
+        try StandaloneApplication.install(f.application, runtimeKey: key, root: f.cache, deadline: f.deadline)
+    }
+}
+
 @Test func aCompetingInstallerDoesNotUseAPartialGeneration() throws {
     let f = try ApplicationFixture(); defer { f.close() }
     let key = try f.key