Context Servers: Protocol fixes and UI improvements (#19087)

This PR does two things. It fixes some minor inconsistencies in the
protocol. This is mostly about handling JSON RPC notifications correctly
and skipping fields when set to None.

Second part is about improving the rendering of context server commands,
by passing on the description
of the command to the slash command UI and showing the name of the
argument as a CodeLabel.

Release Notes:

- N/A

crates/context_servers/src/manager.rs

@@ -85,7 +85,7 @@ impl ContextServer {
         )?;
 
         let protocol = crate::protocol::ModelContextProtocol::new(client);
-        let client_info = types::EntityInfo {
+        let client_info = types::Implementation {
             name: "Zed".to_string(),
             version: env!("CARGO_PKG_VERSION").to_string(),
         };