Non initialized McpServerSession hangs and timesout instead of providing helpful error message

[marianogonzalez]

This issue is closely related to #274 and could be considered as a follow up to that, but I decided to split it for convenience and readability.

Calling McpServerSession#handle(JSonRPCMessage) on an instance that hasn't yet received the notifications/initialized notification results in a Mono that never completes until a Timeout is raised.

This happens because the io.modelcontextprotocol.spec.McpServerSession#exchangeSink will never emit any value until such notification is received, and thus the this.exchangeSink.asMono().flatMap(exchange -> handler.handle(exchange, request.params())); in McpServerSession#225 will never complete.

Putting the considerations of #274 aside, it is fair for a non intiailized session to not handle messages, but it should do it by quickly returning a Mono.error instead of hanging and returning an unspecified timeout.