Actually fix concurrency in tests this time

DiscordChatExporter.Cli.Tests/Infra/ExportWrapper.cs

@@ -1,9 +1,11 @@
 using System;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
 using System.Reflection;
 using System.Text.Json;
+using System.Threading;
 using System.Threading.Tasks;
 using AngleSharp.Dom;
 using AngleSharp.Html.Dom;
@@ -18,6 +20,8 @@ namespace DiscordChatExporter.Cli.Tests.Infra;
 
 public static class ExportWrapper
 {
+    private static readonly ConcurrentDictionary<string, SemaphoreSlim> Locks = new();
+
     private static readonly string DirPath = Path.Combine(
         Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location) ?? Directory.GetCurrentDirectory(),
         "ExportCache"
@@ -37,6 +41,12 @@ public static class ExportWrapper
     }
 
     private static async ValueTask<string> ExportAsync(Snowflake channelId, ExportFormat format)
+    {
+        // Lock separately for each channel and format
+        var channelLock = Locks.GetOrAdd($"{channelId}_{format}", _ => new SemaphoreSlim(1, 1));
+        await channelLock.WaitAsync();
+
+        try
         {
             var fileName = channelId.ToString() + '.' + format.GetFileExtension();
             var filePath = Path.Combine(DirPath, fileName);
@@ -55,6 +65,11 @@ public static class ExportWrapper
 
             return await File.ReadAllTextAsync(filePath);
         }
+        finally
+        {
+            channelLock.Release();
+        }
+    }
 
     public static async ValueTask<IHtmlDocument> ExportAsHtmlAsync(Snowflake channelId) => Html.Parse(
         await ExportAsync(channelId, ExportFormat.HtmlDark)