Add InMemoryTransport

[twenzel]

This PR adds a third server transport: InMemoryServerTransport.

Motivation and Context

It can be used for testing or other special scenarios. Due to the fact, that McpServer requires a registered IServerTransport we need to provide at least a "dumb" one.

[halter73]

Thanks for the PR! It looks like you might have missed some of the conversation in #97 (comment)

Please hold off on more work until we've updated the transport interfaces. The IServerTransport in particular is changing to support multiple sessions. You can expect a PR with the interface changes later today.

[halter73]

#142 is the PR that updates the IServerTransport interface. Please take a look.

[stephentoub]

Thanks for your interest here, @twenzel. @halter73 has changed the interfaces, and I'm currently in the middle of refactoring out stream-based transports from the stdio ones. Given that, I don't think we're going to need separate in-memory transports. I'll close this, and if after that lands we decide we still need something more, we can revisit this.