context_store: Refactor state management (#29910)

Because we instantiated `ContextServerManager` both in `agent` and
`assistant-context-editor`, and these two entities track the running MCP
servers separately, we were effectively running every MCP server twice.

This PR moves the `ContextServerManager` into the project crate (now
called `ContextServerStore`). The store can be accessed via a project
instance. This ensures that we only instantiate one `ContextServerStore`
per project.

Also, this PR adds a bunch of tests to ensure that the
`ContextServerStore` behaves correctly (Previously there were none).

Closes #28714
Closes #29530

Release Notes:

- N/A

crates/extension/src/extension_host_proxy.rs

@@ -362,6 +362,8 @@ pub trait ExtensionContextServerProxy: Send + Sync + 'static {
         server_id: Arc<str>,
         cx: &mut App,
     );
+
+    fn unregister_context_server(&self, server_id: Arc<str>, cx: &mut App);
 }
 
 impl ExtensionContextServerProxy for ExtensionHostProxy {
@@ -377,6 +379,14 @@ impl ExtensionContextServerProxy for ExtensionHostProxy {
 
         proxy.register_context_server(extension, server_id, cx)
     }
+
+    fn unregister_context_server(&self, server_id: Arc<str>, cx: &mut App) {
+        let Some(proxy) = self.context_server_proxy.read().clone() else {
+            return;
+        };
+
+        proxy.unregister_context_server(server_id, cx)
+    }
 }
 
 pub trait ExtensionIndexedDocsProviderProxy: Send + Sync + 'static {