Fixed missing initial preamble in the MultiPart response formmatter.

src/HotChocolate/AspNetCore/test/AspNetCore.Tests/__snapshots__/HttpPostMiddlewareTests.BatchRequest_GetHero_And_GetHuman_MultiPart.snap

@@ -1,3 +1,4 @@
+
 ---
 Content-Type: application/json; charset=utf-8
 

src/HotChocolate/AspNetCore/test/AspNetCore.Tests/__snapshots__/HttpPostMiddlewareTests.Ensure_Multipart_Format_Is_Correct_With_Defer.snap

@@ -1,3 +1,4 @@
+
 ---
 Content-Type: application/json; charset=utf-8
 

src/HotChocolate/AspNetCore/test/AspNetCore.Tests/__snapshots__/HttpPostMiddlewareTests.Ensure_Multipart_Format_Is_Correct_With_Defer_If_Condition_True.snap

@@ -1,3 +1,4 @@
+
 ---
 Content-Type: application/json; charset=utf-8
 

src/HotChocolate/AspNetCore/test/AspNetCore.Tests/__snapshots__/HttpPostMiddlewareTests.Ensure_Multipart_Format_Is_Correct_With_Stream.snap

@@ -1,3 +1,4 @@
+
 ---
 Content-Type: application/json; charset=utf-8
 

src/HotChocolate/AspNetCore/test/AspNetCore.Tests/__snapshots__/HttpPostMiddlewareTests.SingleRequest_Defer_Results.snap

@@ -1,5 +1,5 @@
 {
   "ContentType": "multipart/mixed; boundary=\"-\"",
   "StatusCode": "OK",
-  "Content": "---\r\nContent-Type: application/json; charset=utf-8\r\n\r\n{\"data\":{\"hero\":{\"name\":\"R2-D2\"}},\"hasNext\":true}\r\n---\r\nContent-Type: application/json; charset=utf-8\r\n\r\n{\"label\":\"my_id\",\"path\":[\"hero\"],\"data\":{\"id\":\"2001\"},\"hasNext\":false}\r\n-----\r\n"
+  "Content": "\r\n---\r\nContent-Type: application/json; charset=utf-8\r\n\r\n{\"data\":{\"hero\":{\"name\":\"R2-D2\"}},\"hasNext\":true}\r\n---\r\nContent-Type: application/json; charset=utf-8\r\n\r\n{\"label\":\"my_id\",\"path\":[\"hero\"],\"data\":{\"id\":\"2001\"},\"hasNext\":false}\r\n-----\r\n"
 }

src/HotChocolate/Core/src/Execution/Serialization/MultiPartResponseStreamFormatter.cs

@@ -103,7 +103,7 @@ public sealed partial class MultiPartResponseStreamFormatter : IResponseStreamFo
         await outputStream.FlushAsync(ct).ConfigureAwait(false);
     }
 
-    private async Task WriteResultAsync(
+    private async ValueTask WriteResultAsync(
         IQueryResult result,
         Stream outputStream,
         CancellationToken ct)
@@ -116,10 +116,9 @@ public sealed partial class MultiPartResponseStreamFormatter : IResponseStreamFo
         // The payload is sent, followed by a CRLF.
         var buffer = writer.GetInternalBuffer();
         await outputStream.WriteAsync(buffer, 0, writer.Length, ct).ConfigureAwait(false);
-        await outputStream.WriteAsync(CrLf, 0, CrLf.Length, ct).ConfigureAwait(false);
     }
 
-    private static async Task WriteResultHeaderAsync(
+    private static async ValueTask WriteResultHeaderAsync(
         Stream outputStream,
         CancellationToken ct)
     {
@@ -134,21 +133,23 @@ public sealed partial class MultiPartResponseStreamFormatter : IResponseStreamFo
         await outputStream.WriteAsync(CrLf, 0, CrLf.Length, ct).ConfigureAwait(false);
     }
 
-    private static async Task WriteNextAsync(
+    private static async ValueTask WriteNextAsync(
         Stream outputStream,
         CancellationToken ct)
     {
-        // Each part of the multipart response must start with --- and a CRLF
+        // Before each part of the multi-part response, a boundary (CRLF, ---, CRLF) is sent.
+        await outputStream.WriteAsync(CrLf, 0, CrLf.Length, ct).ConfigureAwait(false);
         await outputStream.WriteAsync(Start, 0, Start.Length, ct).ConfigureAwait(false);
         await outputStream.WriteAsync(CrLf, 0, CrLf.Length, ct).ConfigureAwait(false);
     }
 
-    private static async Task WriteEndAsync(
+    private static async ValueTask WriteEndAsync(
         Stream outputStream,
         CancellationToken ct)
     {
-        // After the last part of the multipart response is sent, the terminating
-        // boundary ----- is sent, followed by a CRLF
+        // After the final payload, the terminating boundary of CRLF followed by
+        // ----- followed by CRLF is sent.
+        await outputStream.WriteAsync(CrLf, 0, CrLf.Length, ct).ConfigureAwait(false);
         await outputStream.WriteAsync(End, 0, End.Length, ct).ConfigureAwait(false);
         await outputStream.WriteAsync(CrLf, 0, CrLf.Length, ct).ConfigureAwait(false);
     }