Fix string escaping problem

client/src/main/scala/caliban/client/__Value.scala

@@ -30,7 +30,7 @@ object __Value {
     override def toString: String = value
   }
   case class __StringValue(value: String)                   extends __Value {
-    override def toString: String = s""""${value.replace("\"", "\\\"")}""""
+    override def toString: String = Json.fromString(value).toString
   }
   case class __BooleanValue(value: Boolean)                 extends __Value {
     override def toString: String = value.toString

client/src/test/scala/caliban/client/ArgEncoderSpec.scala

@@ -0,0 +1,25 @@
+package caliban.client
+
+import zio.test.Assertion.equalTo
+import zio.test._
+import zio.test.environment.TestEnvironment
+
+object ArgEncoderSpec extends DefaultRunnableSpec {
+  override def spec: ZSpec[TestEnvironment, Any] =
+    suite("ArgEncoderSpec")(
+      suite("__StringValue")(
+        test("regular string") {
+          assert(ArgEncoder.string.encode("abcde who am i?").toString)(equalTo(""""abcde who am i?""""))
+        },
+        test("string with quotes") {
+          assert(ArgEncoder.string.encode("abcde \"who am i?\"").toString)(equalTo(""""abcde \"who am i?\"""""))
+        },
+        test("string with new line") {
+          assert(ArgEncoder.string.encode("abcde\n who\n am\n i\n").toString)(equalTo(""""abcde\n who\n am\n i\n""""))
+        },
+        test("string with null characters") {
+          assert(ArgEncoder.string.encode("abcde who am i\u0000").toString)(equalTo("\"abcde who am i\\u0000\""))
+        }
+      )
+    )
+}