Improve code quality: remove unused code, simplify random, dynamic timestamps

- Remove unused buildToolCalls() function (logic is inlined)\n- Simplify randomString() using global rand (Go 1.20+)\n- Use dynamic timestamps instead of hardcoded 1234567890\n- Add time import to converter.go

handler.go

@@ -253,9 +253,8 @@ func writeError(w http.ResponseWriter, code int, message, errType, errCode strin
 func randomString(n int) string {
 	const letters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
 	b := make([]byte, n)
-	r := rand.New(rand.NewSource(time.Now().UnixNano()))
 	for i := range b {
-		b[i] = letters[r.Intn(len(letters))]
+		b[i] = letters[rand.Intn(len(letters))]
 	}
 	return string(b)
 }