context_server: Abstract server transport (#24528)

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

---------

Co-authored-by: Marshall Bowers <git@maxdeviant.com>

crates/context_server/src/context_server.rs

@@ -4,6 +4,7 @@ mod extension_context_server;
 pub mod manager;
 pub mod protocol;
 mod registry;
+mod transport;
 pub mod types;
 
 use command_palette_hooks::CommandPaletteFilter;