Allow tools to continue running when session is closed

Is your feature request related to a problem? Please describe.

A server may implement a long-running tool that should continue running if the client disconnects or session is closed due to idle timeout.

The MCP Spec permits this case and has provisions for reconnecting to the server to resume its connection to the tool (resumable streams), but this is currently not possible with the C# SDK because it terminates tool calls when the session is closed.

Describe the solution you'd like

An option in the McpServerOptions, or maybe in the HttpTransport options, or maybe on individual tools, that skip cancellation of outstanding tool calls when a session is closed.

Describe alternatives you've considered

An alternative might be to not close a session when the client disconnects, and/or disable the idle session timeout.

まず、C# SDK でセッションの終了とアイドルタイムアウトが保留中のツール呼び出しをどのようにキャンセルするかを追跡します。McpServerOptions、HttpTransport のオプション、個々のツールの設定を、MCP の再開可能なストリームに関する規定と比較します。切断またはセッション終了後も対象となるツール呼び出しを保持し、それらへの再接続をサポートする文書化されたオプションがあれば完了です。