Fix and enhance cancellation operations across MCP Sessions. (#179)

* 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>

src/ModelContextProtocol/Server/McpServerOptions.cs

@@ -1,6 +1,5 @@
 
 using ModelContextProtocol.Protocol.Types;
-using System.Text.Json.Serialization;
 
 namespace ModelContextProtocol.Server;