assistant2: Add modal for adding context servers (#27434)

This PR adds a modal for adding context servers from the Assistant 2
configuration view:

<img width="1394" alt="Screenshot 2025-03-25 at 12 22 50 PM"
src="https://github.com/user-attachments/assets/52fe194f-7d88-4f3b-aee1-8c6385136e6b"
/>

Release Notes:

- N/A

crates/assistant2/src/assistant.rs

@@ -32,6 +32,7 @@ use prompt_store::PromptBuilder;
 use settings::Settings as _;
 
 pub use crate::active_thread::ActiveThread;
+use crate::assistant_configuration::AddContextServerModal;
 pub use crate::assistant_panel::{AssistantPanel, ConcreteAssistantPanelDelegate};
 pub use crate::inline_assistant::InlineAssistant;
 pub use crate::thread::{Message, RequestKind, Thread, ThreadEvent};
@@ -46,6 +47,7 @@ actions!(
         RemoveAllContext,
         OpenHistory,
         OpenConfiguration,
+        AddContextServer,
         RemoveSelectedThread,
         Chat,
         ChatMode,
@@ -86,6 +88,7 @@ pub fn init(
         client.telemetry().clone(),
         cx,
     );
+    cx.observe_new(AddContextServerModal::register).detach();
 
     feature_gate_assistant2_actions(cx);
 }