assistant panel: Stop animation & show explicit state if canceled (#16200)

This fixes a bug by stopping the animation when a completion is canceled
and it also makes the state more explicit, which I think is very
valuable.



https://github.com/user-attachments/assets/9ede9b25-86ac-4901-8434-7407896bb799


Release Notes:

- N/A

crates/proto/proto/zed.proto

@@ -2310,6 +2310,7 @@ message ContextMessageStatus {
         Done done = 1;
         Pending pending = 2;
         Error error = 3;
+        Canceled canceled = 4;
     }
 
     message Done {}
@@ -2319,6 +2320,8 @@ message ContextMessageStatus {
     message Error {
         string message = 1;
     }
+
+    message Canceled {}
 }
 
 message ContextMessage {