Fix race in TransportBase.SetConnected

[halter73]

• Previously the ChannelWriter could already be completed if multiple threads raced in SetConnected(false)

  Failed ModelContextProtocol.Tests.ClientIntegrationTests.CallTool_Stdio_EchoServer(clientId: "test_server") [331 ms]
  Error Message:
   System.Threading.Channels.ChannelClosedException : The channel has been closed.
  Stack Trace:
     at System.Threading.Channels.ChannelWriter`1.Complete(Exception error)
   at ModelContextProtocol.Protocol.Transport.TransportBase.SetConnected(Boolean isConnected) in /_/src/ModelContextProtocol/Protocol/Transport/TransportBase.cs:line 75
   at ModelContextProtocol.Protocol.Transport.StreamClientSessionTransport.CleanupAsync(CancellationToken cancellationToken) in /_/src/ModelContextProtocol/Protocol/Transport/StreamClientSessionTransport.cs:line 192
   at ModelContextProtocol.Client.McpClient.DisposeUnsynchronizedAsync() in /_/src/ModelContextProtocol/Client/McpClient.cs:line 184
   at ModelContextProtocol.Shared.McpEndpoint.DisposeAsync() in /_/src/ModelContextProtocol/Shared/McpEndpoint.cs:line 89
   at ModelContextProtocol.Tests.ClientIntegrationTests.CallTool_Stdio_EchoServer(String clientId) in /_/tests/ModelContextProtocol.Tests/ClientIntegrationTests.cs:line 95
--- End of stack trace from previous location ---
  Standard Output Messages:
 | [2025-04-09T16:35:33] ModelContextProtocol.Client.McpClientFactory Information: Creating client for TestServer
 | [2025-04-09T16:35:33] ModelContextProtocol.Client.McpClient Information: Sending request for TestServer with method initialize
 | [2025-04-09T16:35:33] ModelContextProtocol.Client.McpClient Information: Request response received payload for TestServer: {"protocolVersion":"2024-11-05","capabilities":{"logging":{},"prompts":{},"resources":{"subscribe":true},"tools":{},"completions":{}},"serverInfo":{"name":"TestServer","version":"1.0.0.0"},"instructions":"This is a test server with only stub functionality"}
 | [2025-04-09T16:35:33] ModelContextProtocol.Client.McpClient Information: Request response received for TestServer with method initialize
 | [2025-04-09T16:35:33] ModelContextProtocol.Client.McpClient Information: Server TestServer capabilities received: {"logging":{},"prompts":{},"resources":{"subscribe":true},"tools":{},"completions":{}}, server info: {"name":"TestServer","version":"1.0.0.0"}
 | [2025-04-09T16:35:33] ModelContextProtocol.Client.McpClientFactory Information: Client for TestServer created and connected
 | [2025-04-09T16:35:33] ModelContextProtocol.Client.McpClient Information: Sending request for TestServer with method tools/call
 | [2025-04-09T16:35:33] ModelContextProtocol.Client.McpClient Information: Request response received payload for TestServer: {"content":[{"type":"text","text":"Echo: Hello MCP!"}],"isError":false}
 | [2025-04-09T16:35:33] ModelContextProtocol.Client.McpClient Information: Request response received for TestServer with method tools/call
 | [2025-04-09T16:35:33] ModelContextProtocol.Client.McpClient Information: Cleaning up endpoint TestServer
 | [2025-04-09T16:35:33] ModelContextProtocol.Client.McpClient Information: Endpoint message processing cancelled for TestServer
 | [2025-04-09T16:35:33] ModelContextProtocol.Client.McpClient Information: Endpoint cleaned up for TestServer

https://github.com/modelcontextprotocol/csharp-sdk/actions/runs/14362374601/job/40266898759?pr=252