refactor(macos): extract gateway payload decoding

apps/macos/Sources/Clawdis/GatewayPayloadDecoding.swift

@@ -0,0 +1,16 @@
+import ClawdisProtocol
+import Foundation
+
+enum GatewayPayloadDecoding {
+    static func decode<T: Decodable>(_ payload: ClawdisProtocol.AnyCodable, as _: T.Type = T.self) throws -> T {
+        let data = try JSONEncoder().encode(payload)
+        return try JSONDecoder().decode(T.self, from: data)
+    }
+
+    static func decodeIfPresent<T: Decodable>(_ payload: ClawdisProtocol.AnyCodable?, as _: T.Type = T.self) throws
+        -> T?
+    {
+        guard let payload else { return nil }
+        return try decode(payload, as: T.self)
+    }
+}