Avoid race starting SseResponseStreamTransport #91
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The logging from https://github.com/modelcontextprotocol/csharp-sdk/actions/runs/14040589658/job/39309497411 indicates that one source of test flakiness is a real product issue.
_sseWriteTaskmight not be set until after the SSE endpoint event has already been sent sinceSseFormatter.WriteAsyncdoes not return a task until it awaits. This means that it's possible for the client to find the "/message" endpoint and send a message to that beforeIsConnectedis true which can lead to an improperInvalidOperationExceptionas seen in the logs.