fix: Windows-Kompatibilität - Streaming, Execute-Deadlock, Plan-Modus bei Choice

- HttpResponseMessage mit using disposen (3 Stellen in AnthropicService)
  verhindert Socket-Exhaustion unter Windows (2 Connections/Host Limit)
- SSE DisableBuffering() in ChatEndpoint für sofortigen Flush unter Windows
- Execute: stdout/stderr vor WaitForExitAsync lesen statt danach
  verhindert Pipe-Deadlock bei vollem Buffer (Windows: 4KB)
- Plan-Modus: Frontend sendet planMode bei Choice-Response mit
  Backend-Fallback auf vorherigen Chain entfernt - User-Entscheidung gilt

Endpoints/ChatEndpoint.cs

@@ -52,19 +52,7 @@ public static class ChatEndpoint
         var userId = req.HttpContext.GetUserId();
         var maxIterations = config.GetValue<int>("MaxIterations", 100);
 
-        // Bei Choice-Response: planMode vom vorherigen Chain übernehmen
-        var effectivePlanMode = body.PlanMode;
-        if (isChoiceResponse && !effectivePlanMode)
-        {
-            var prevChainId = await chatService.FindChainWithToolCall(sessionId, body.ChoiceToolCallId);
-            if (prevChainId != null)
-            {
-                var prevChain = await db.GetChain(prevChainId);
-                if (prevChain != null) effectivePlanMode = prevChain.PlanMode;
-            }
-        }
-
-        var ctx = await chatService.PrepareContext(sessionId, userId, maxIterations, planMode: effectivePlanMode);
+        var ctx = await chatService.PrepareContext(sessionId, userId, maxIterations, planMode: body.PlanMode);
         if (ctx == null)
         {
             res.StatusCode = 400;
@@ -132,6 +120,10 @@ public static class ChatEndpoint
         res.Headers["Cache-Control"] = "no-cache";
         res.Headers["Connection"] = "keep-alive";
 
+        // Response-Buffering deaktivieren – wichtig für SSE unter Windows
+        var bufferingFeature = res.HttpContext.Features.Get<Microsoft.AspNetCore.Http.Features.IHttpResponseBodyFeature>();
+        bufferingFeature?.DisableBuffering();
+
         // ── Preparer-Agent Phase (vor dem Worker) ──
         if (ctx.Preparer != null && !isChoiceResponse)
         {