Improve context server lifecycle management (#20622)

This optimizes and fixes bugs in our logic for maintaining a set of
running context servers, based on the combination of the user's
`context_servers` settings and their installed extensions.

Release Notes:

- N/A

---------

Co-authored-by: Marshall <marshall@zed.dev>
Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/assistant/src/slash_command/context_server_command.rs

@@ -27,7 +27,7 @@ pub struct ContextServerSlashCommand {
 impl ContextServerSlashCommand {
     pub fn new(
         server_manager: Model<ContextServerManager>,
-        server: &Arc<dyn ContextServer>,
+        server: &Arc<ContextServer>,
         prompt: Prompt,
     ) -> Self {
         Self {