* Add idle session pruning tests
* Remove redundant CTS from StreamableHttpHandler
* StreamableHttpServerTransport.HandlePostRequest already creates the equivelant CTS
which DeleteRequest_CompletesSession_WhichCancelsLongRunningToolCalls verifies
* Fix accept header validation for GET
* Use InvalidOperationException for transport-not-connected errors in doc comments
* Fix flaky Cancellation_ThrowsCancellationException test