Watch: treat an empty threadId in a push as no thread

The gateway sends threadId "" on host notifications; tapping one now opens
the feed instead of a thread that does not exist.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

watch/WristCode/Notifications/NotificationHandler.swift

@@ -20,9 +20,13 @@ enum NotificationCommand: Equatable, Sendable {
 
     /// `userInfo` holds the custom keys `hostId`, `threadId` and, for approvals, `requestId`.
     init(actionIdentifier: String, userInfo: [AnyHashable: Any], text: String?) {
-        let hostId = userInfo["hostId"] as? String
-        let threadId = userInfo["threadId"] as? String
-        let requestId = userInfo["requestId"] as? String
+        // The gateway sends `threadId: ""` on host notifications, so empty means absent.
+        func value(_ key: String) -> String? {
+            (userInfo[key] as? String).flatMap { $0.isEmpty ? nil : $0 }
+        }
+        let hostId = value("hostId")
+        let threadId = value("threadId")
+        let requestId = value("requestId")
         let thread: Route? = if let hostId, let threadId { .thread(hostId: hostId, threadId: threadId) } else { nil }
 
         switch actionIdentifier {

watch/WristCodeTests/ModelsTests.swift

@@ -198,6 +198,8 @@ private let feedItemJSON = """
         // A host notification has no thread, so tapping it opens the feed.
         #expect(NotificationCommand(actionIdentifier: "com.apple.UNNotificationDefaultActionIdentifier", userInfo: [:], text: nil)
             == .open(nil))
+        #expect(NotificationCommand(actionIdentifier: "com.apple.UNNotificationDefaultActionIdentifier", userInfo: ["hostId": "h", "threadId": ""], text: nil)
+            == .open(nil))
         // Allow without a requestId cannot answer, so it opens the thread instead.
         #expect(NotificationCommand(actionIdentifier: "allow", userInfo: ["hostId": "h", "threadId": "t"], text: nil)
             == .open(.thread(hostId: "h", threadId: "t")))