Fix SendRequestAsync hang when background SSE stream delivers response before foreground transport send completes#1381

Copilot wants to merge 4 commits intomainmodelcontextprotocol/csharp-sdk:mainfrom

copilot/fix-race-condition-mcpsessionhandlermodelcontextprotocol/csharp-sdk:copilot/fix-race-condition-mcpsessionhandlerCopy head branch name to clipboard