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/Cargo.toml

@@ -14,6 +14,7 @@ path = "src/context_server.rs"
 [dependencies]
 anyhow.workspace = true
 assistant_tool.workspace = true
+async-trait.workspace = true
 collections.workspace = true
 command_palette_hooks.workspace = true
 context_server_settings.workspace = true