fix: prevent double-close errors on tcp handle

When a connection is interrupted during an async write, the callback
can attempt to close a handle that's already closing, causing:
  handle 0x... is already closing

tcp.lua:161-162 correctly guards with is_closing():
  if not client.tcp_handle:is_closing() then
    client.tcp_handle:close()
  end

But client.lua:219,223 was missing this guard. Added the same defensive
check to prevent the race condition.

Author: Vvkmnn

lua/claudecode/server/client.lua

@@ -216,11 +216,15 @@ function M.close_client(client, code, reason)
     local close_frame = frame.create_close_frame(code, reason)
     client.tcp_handle:write(close_frame, function()
       client.state = "closed"
-      client.tcp_handle:close()
+      if not client.tcp_handle:is_closing() then
+        client.tcp_handle:close()
+      end
     end)
   else
     client.state = "closed"
-    client.tcp_handle:close()
+    if not client.tcp_handle:is_closing() then
+      client.tcp_handle:close()
+    end
   end
 
   client.state = "closing"