Expose pinned messages through iOS FFI

scripts/smoke-ios-ffi-swift.sh

@@ -60,6 +60,9 @@ struct Smoke {
         let history = try session.loadHistory(chatId: chat.id, limit: 20)
         require(history.first?.text == "Hello from fake TDLib", "expected seeded history")
 
+        let pinned = try session.pinnedMessages(chatId: chat.id)
+        require(pinned.first?.text == "Hello from fake TDLib", "expected pinned message")
+
         let sent = try session.sendMessage(chatId: chat.id, text: "Hi from Swift FFI", replyToMessageId: nil)
         require(sent.text == "Hi from Swift FFI", "expected sent message text")