JsonRpc: write frame header straight into the pooled buffer

BuildFrame stackalloc'd a header scratch then copied it into the rented
frame. Over-rent by the fixed <=30B header bound instead and write the header
directly into the frame, dropping the scratch buffer and header copy. The
single JSON copy remains (its length must precede it as Content-Length).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: SteveSandersonMS

dotnet/src/JsonRpc.cs

@@ -241,14 +241,16 @@ private static byte[] BuildFrame(ReadOnlySpan<byte> json, out int frameLen)
         // "Content-Length: " (16) + max int digits (10) + "\r\n\r\n" (4)
         const int MaxHeaderLength = 30;
 
-        Span<byte> header = stackalloc byte[MaxHeaderLength];
-        bool wrote = Utf8.TryWrite(header, $"Content-Length: {json.Length}\r\n\r\n", out int headerLen);
+        // Over-rent by the (fixed, tiny) header bound so the header can be written
+        // straight into the frame — no scratch buffer or header copy. The JSON is
+        // already UTF-8, so the only copy is placing it after the header, which is
+        // unavoidable since Content-Length needs its length up front.
+        var frame = ArrayPool<byte>.Shared.Rent(MaxHeaderLength + json.Length);
+        bool wrote = Utf8.TryWrite(frame, $"Content-Length: {json.Length}\r\n\r\n", out int headerLen);
         Debug.Assert(wrote && headerLen > 0);
 
-        frameLen = headerLen + json.Length;
-        var frame = ArrayPool<byte>.Shared.Rent(frameLen);
-        header.Slice(0, headerLen).CopyTo(frame);
         json.CopyTo(frame.AsSpan(headerLen));
+        frameLen = headerLen + json.Length;
         return frame;
     }