context_server: Abstract server transport

[fdionisi]

This PR abstracts the communication layer for context servers, laying the groundwork for supporting multiple transport mechanisms and taking one step towards enabling remote servers.

Key changes centre around creating a new Transport trait with methods for sending and receiving messages. I've implemented this trait for the existing stdio-based communication, which is now encapsulated in a StdioTransport struct. The Client struct has been refactored to use this new Transport trait instead of directly managing stdin and stdout.

The next steps will involve implementing an SSE + HTTP transport and defining alternative context server settings for remote servers.

Release Notes:

• N/A

[fdionisi]

Hey @maxdeviant, thank you for taking over - these are changes I had in my mind, too, but I wanted to validate the direction before committing to it.

I went ahead on a separate branch to implement SSE transport, but I'm hitting a wall when trying to test it. For some reason, context servers are not getting initialised anymore on dev builds, and I cannot understand what I am doing wrong.
I'd love to get a draft PR up and have another pair of eyes looking at it, time permitting. What's the best way to move forward?