Send DELETE request when closing an Streamable HTTP session on the server

[halter73]

StreamableHttpClientSessionTransport.DisposeAsync should be updated to send a delete request. We should ignore non-successful status code because the MCP spec does not require that servers handle the DELETE request, but we shouldn't catch any exceptions.

5. Clients that no longer need a particular session (e.g., because the user is leaving the client application) SHOULD send an HTTP DELETE to the MCP endpoint with the Mcp-Session-Id header, to explicitly terminate the session.

• The server MAY respond to this request with HTTP 405 Method Not Allowed, indicating that the server does not allow clients to terminate sessions.

https://modelcontextprotocol.io/specification/2025-03-26/basic/transports#streamable-http

Update the StreamableHttpClientConformanceTests to verify that this DELETE request gets sent.