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/extension_host/src/extension_host.rs

@@ -141,7 +141,7 @@ pub trait ExtensionRegistrationHooks: Send + Sync + 'static {
         &self,
         _id: Arc<str>,
         _extension: WasmExtension,
-        _host: Arc<WasmHost>,
+        _cx: &mut AppContext,
     ) {
     }
 
@@ -1266,7 +1266,7 @@ impl ExtensionStore {
                         this.registration_hooks.register_context_server(
                             id.clone(),
                             wasm_extension.clone(),
-                            this.wasm_host.clone(),
+                            cx,
                         );
                     }