Stream asset downloads to avoid OutOfMemoryException on large files

ExportAssetDownloader used HttpClient.GetAsync(url, ct), which defaults to
HttpCompletionOption.ResponseContentRead and buffers the entire response body
into memory before returning. Downloading a large attachment therefore tried to
grow a single in-memory buffer to the full file size and threw
OutOfMemoryException (HttpContent.LoadIntoBufferAsync -> GrowAndWrite).

Pass HttpCompletionOption.ResponseHeadersRead so the body is streamed straight
to disk via CopyToAsync in bounded chunks, matching how DiscordClient already
issues its requests.

DiscordChatExporter.Core/Exporting/ExportAssetDownloader.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.IO;
+using System.Net.Http;
 using System.Security.Cryptography;
 using System.Text;
 using System.Threading;
@@ -63,8 +64,15 @@ internal partial class ExportAssetDownloader(string workingDirPath, bool reuse)
         await Http.ResiliencePipeline.ExecuteAsync(
             async innerCancellationToken =>
             {
-                // Download the file
+                // Download the file.
-                using var response = await Http.Client.GetAsync(url, innerCancellationToken);
+                // Use ResponseHeadersRead so the response body is streamed to disk instead of
+                // being fully buffered into memory first, which can cause an OutOfMemoryException
+                // on large attachments.
+                using var response = await Http.Client.GetAsync(
+                    url,
+                    HttpCompletionOption.ResponseHeadersRead,
+                    innerCancellationToken
+                );
                 await using var output = File.Create(filePath);
                 await response.Content.CopyToAsync(output, innerCancellationToken);
             },