fix(GODT-2424): Sync Builder Message Split

Incorrect math was causing some messages to not be built and be missing
from Gluon.

internal/user/sync_build_test.go

@@ -24,6 +24,8 @@ import (
 
 	"github.com/ProtonMail/gluon/imap"
 	"github.com/ProtonMail/gluon/rfc822"
+	"github.com/ProtonMail/go-proton-api"
+	"github.com/bradenaw/juniper/xslices"
 	"github.com/stretchr/testify/require"
 )
 
@@ -47,3 +49,32 @@ func TestNewFailedMessageLiteral(t *testing.T) {
 	require.Equal(t, `("text" "plain" () NIL NIL "base64" 114 2)`, parsed.Body)
 	require.Equal(t, `("text" "plain" () NIL NIL "base64" 114 2 NIL NIL NIL NIL)`, parsed.Structure)
 }
+
+func TestSyncChunkSyncBuilderBatch(t *testing.T) {
+	// GODT-2424 - Some messages were not fully built due to a bug in the chunking if the total memory used by the
+	// message would be higher than the maximum we allowed.
+	const totalMessageCount = 100
+
+	msg := proton.FullMessage{
+		Message: proton.Message{
+			Attachments: []proton.Attachment{
+				{
+					Size: int64(8 * Megabyte),
+				},
+			},
+		},
+		AttData: nil,
+	}
+
+	messages := xslices.Repeat(msg, totalMessageCount)
+
+	chunks := chunkSyncBuilderBatch(messages, 16*Megabyte)
+
+	var totalMessagesInChunks int
+
+	for _, v := range chunks {
+		totalMessagesInChunks += len(v)
+	}
+
+	require.Equal(t, totalMessagesInChunks, totalMessageCount)
+}