* 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
* Propagate CancellationToken request cancellation to remote endpoint
* Refactor existing tests to have a shared base class
* Add cancellation tests
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Co-authored-by: Stephen Toub <stoub@microsoft.com>