Prevent GET SSE request failure from causing client disposal exception

Example failure caused by this:

  Failed ModelContextProtocol.AspNetCore.Tests.MapMcpStatelessTests.Server_ShutsDownQuickly_WhenClientIsConnected [138 ms]
  Error Message:
   System.Net.Http.HttpRequestException : An error occurred while sending the request.
---- System.InvalidOperationException : Reading is not allowed after reader was completed.
  Stack Trace:
     at System.Net.Http.HttpConnection.SendAsync(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken)
   at System.Net.Http.HttpConnectionPool.SendWithVersionDetectionAndRetryAsync(HttpRequestMessage request, Boolean async, Boolean doRequestAuth, CancellationToken cancellationToken)
   at System.Net.Http.RedirectHandler.SendAsync(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken)
   at System.Net.Http.HttpClient.<SendAsync>g__Core|83_0(HttpRequestMessage request, HttpCompletionOption completionOption, CancellationTokenSource cts, Boolean disposeCts, CancellationTokenSource pendingRequestsCts, CancellationToken originalCancellationToken)
   at ModelContextProtocol.Client.McpHttpClient.SendAsync(HttpRequestMessage request, JsonRpcMessage message, CancellationToken cancellationToken) in /_/src/ModelContextProtocol.Core/Client/McpHttpClient.cs:line 22
   at ModelContextProtocol.Client.StreamableHttpClientSessionTransport.ReceiveUnsolicitedMessagesAsync() in /_/src/ModelContextProtocol.Core/Client/StreamableHttpClientSessionTransport.cs:line 178
   at ModelContextProtocol.Client.StreamableHttpClientSessionTransport.DisposeAsync() in /_/src/ModelContextProtocol.Core/Client/StreamableHttpClientSessionTransport.cs:line 149
   at ModelContextProtocol.Client.McpClientImpl.DisposeAsync() in /_/src/ModelContextProtocol.Core/Client/McpClientImpl.cs:line 235
   at ModelContextProtocol.AspNetCore.Tests.MapMcpTests.Server_ShutsDownQuickly_WhenClientIsConnected() in /_/tests/ModelContextProtocol.AspNetCore.Tests/MapMcpTests.cs:line 238
   at ModelContextProtocol.AspNetCore.Tests.MapMcpTests.Server_ShutsDownQuickly_WhenClientIsConnected() in /_/tests/ModelContextProtocol.AspNetCore.Tests/MapMcpTests.cs:line 238
--- End of stack trace from previous location ---
----- Inner Stack Trace -----
   at System.IO.Pipelines.ThrowHelper.ThrowInvalidOperationException_NoReadingAllowed()
   at System.IO.Pipelines.Pipe.AdvanceReader(SequencePosition& consumed, SequencePosition& examined)
   at System.IO.Pipelines.PipeReaderStream.HandleReadResult(ReadResult result, Span`1 buffer)
   at System.IO.Pipelines.PipeReaderStream.HandleReadResult(ReadResult result, Span`1 buffer)
   at System.IO.Pipelines.PipeReaderStream.ReadAsyncInternal(Memory`1 buffer, CancellationToken cancellationToken)
   at ModelContextProtocol.AspNetCore.Tests.Utils.KestrelInMemoryConnection.DuplexStream.ReadAsync(Memory`1 buffer, CancellationToken cancellationToken) in /_/tests/ModelContextProtocol.AspNetCore.Tests/Utils/KestrelInMemoryConnection.cs:line 81
   at System.Net.Http.HttpConnection.SendAsync(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken)

Author: halter73

src/ModelContextProtocol.Core/Client/StreamableHttpClientSessionTransport.cs

@@ -175,16 +175,27 @@ private async Task ReceiveUnsolicitedMessagesAsync()
         request.Headers.Accept.Add(s_textEventStreamMediaType);
         CopyAdditionalHeaders(request.Headers, _options.AdditionalHeaders, SessionId, _negotiatedProtocolVersion);
 
-        using var response = await _httpClient.SendAsync(request, message: null, _connectionCts.Token).ConfigureAwait(false);
-
-        if (!response.IsSuccessStatusCode)
+        // Server support for the GET request is optional. If it fails, we don't care. It just means we won't receive unsolicited messages.
+        HttpResponseMessage response;
+        try
+        {
+            response = await _httpClient.SendAsync(request, message: null, _connectionCts.Token).ConfigureAwait(false);
+        }
+        catch (HttpRequestException)
         {
-            // Server support for the GET request is optional. If it fails, we don't care. It just means we won't receive unsolicited messages.
             return;
         }
 
-        using var responseStream = await response.Content.ReadAsStreamAsync(_connectionCts.Token).ConfigureAwait(false);
-        await ProcessSseResponseAsync(responseStream, relatedRpcRequest: null, _connectionCts.Token).ConfigureAwait(false);
+        using (response)
+        {
+            if (!response.IsSuccessStatusCode)
+            {
+                return;
+            }
+
+            using var responseStream = await response.Content.ReadAsStreamAsync(_connectionCts.Token).ConfigureAwait(false);
+            await ProcessSseResponseAsync(responseStream, relatedRpcRequest: null, _connectionCts.Token).ConfigureAwait(false);
+        }
     }
 
     private async Task<JsonRpcMessageWithId?> ProcessSseResponseAsync(Stream responseStream, JsonRpcRequest? relatedRpcRequest, CancellationToken cancellationToken)