Improve client test reliability and execution time

This change uses VirtualTimeScheduler and pretends enough time has
passed to trigger a timeout on the initialization.
Another problem with reliability of the tests was that the used
testcontainer for the SSE server does not support multiple clients and
the existence of both the global client for the entire suite and some
customized local clients in some tests caused responses to be delivered
to the other client at some racing situations. Now each test creates a
dedicated client and performs cleanup locally.
While these tests were improved, two other issues were found and fixed.
The first one is that the closeGracefully of DefaultMcpSession was not
lazy and would trigger connection disposal before the returned Mono was
subscribed. The second one was dealing with closing the StdIo client
before the process was started. In such a case there should not be an
error but rather a warning and successful completion.