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

Services/AnthropicService.cs

@@ -92,7 +92,7 @@ public class AnthropicService
                 LogPayload(json, sessionId, chainId, iterations);
                 var httpContent = new StringContent(json, Encoding.UTF8, "application/json");
 
-                var httpResp = await client.SendAsync(
+                using var httpResp = await client.SendAsync(
                     new HttpRequestMessage(HttpMethod.Post, "https://api.anthropic.com/v1/messages") { Content = httpContent },
                     HttpCompletionOption.ResponseHeadersRead,
                     cancellationToken
@@ -665,7 +665,7 @@ public class AnthropicService
             LogPayload(json, sessionId, chainId, baseIterations + iterations);
             var httpContent = new StringContent(json, Encoding.UTF8, "application/json");
 
-            var httpResp = await client.SendAsync(
+            using var httpResp = await client.SendAsync(
                 new HttpRequestMessage(HttpMethod.Post, "https://api.anthropic.com/v1/messages") { Content = httpContent },
                 cancellationToken: cancellationToken);
 
@@ -873,7 +873,7 @@ public class AnthropicService
                 LogPayload(json, ctx.SessionId, ctx.ChainId, iterations);
                 var httpContent = new StringContent(json, Encoding.UTF8, "application/json");
 
-                var httpResp = await client.SendAsync(
+                using var httpResp = await client.SendAsync(
                     new HttpRequestMessage(HttpMethod.Post, "https://api.anthropic.com/v1/messages") { Content = httpContent });
 
                 var respBody = await httpResp.Content.ReadAsStringAsync();

Services/ToolService.cs

@@ -388,6 +388,10 @@ public class ToolService
                 // Timeout (180s) UND externen CancellationToken verknüpfen
                 using var cts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
                 cts.CancelAfter(TimeSpan.FromSeconds(180));
+                // stdout/stderr VOR WaitForExit lesen – verhindert Deadlock
+                // wenn Pipe-Buffer volllaufen (Windows: nur 4KB!)
+                var stdoutTask = proc.StandardOutput.ReadToEndAsync();
+                var stderrTask = proc.StandardError.ReadToEndAsync();
                 try
                 {
                     await proc.WaitForExitAsync(cts.Token);
@@ -399,8 +403,8 @@ public class ToolService
                     cancellationToken.ThrowIfCancellationRequested();
                     return "Fehler: Timeout nach 180 Sekunden - Prozess abgebrochen";
                 }
-                var stdout = await proc.StandardOutput.ReadToEndAsync();
-                var stderr = await proc.StandardError.ReadToEndAsync();
+                var stdout = await stdoutTask;
+                var stderr = await stderrTask;
                 var result = new StringBuilder();
                 if (!string.IsNullOrEmpty(stdout)) result.Append(stdout);
                 if (!string.IsNullOrEmpty(stderr))