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();