Multipart subs: add trailing data test (#6646)

Author: martinbonnin

tests/multipart/src/commonTest/kotlin/test/MultipartSubscriptionsMockServerTest.kt

@@ -263,6 +263,18 @@ class MultipartSubscriptionsMockServerTest {
     }
   }
 
+  @Test
+  fun trailingDataIsIgnored() = multipartSubsTest {
+    enqueue(
+        "\r\n--graphql\r\nContent-Type: application/json\r\n\r\n{\"payload\":{\"data\":{\"hello\":\"world\"}}}" +
+            "\r\n--graphql--foo\r\nbar"
+    )
+    apolloClient.subscription(HelloSubscription()).toFlow().test(timeout = 300.milliseconds) {
+      assertEquals("world", awaitItem().dataOrThrow().hello)
+      awaitComplete()
+    }
+  }
+
   @Test
   fun malformedMultipartTriggersReturnsErrorResponse() = multipartSubsTest {
     // No end boundary