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/assistant/src/assistant.rs

@@ -102,6 +102,7 @@ pub enum MessageStatus {
     Pending,
     Done,
     Error(SharedString),
+    Canceled,
 }
 
 impl MessageStatus {
@@ -112,6 +113,7 @@ impl MessageStatus {
             Some(proto::context_message_status::Variant::Error(error)) => {
                 MessageStatus::Error(error.message.into())
             }
+            Some(proto::context_message_status::Variant::Canceled(_)) => MessageStatus::Canceled,
             None => MessageStatus::Pending,
         }
     }
@@ -135,6 +137,11 @@ impl MessageStatus {
                     },
                 )),
             },
+            MessageStatus::Canceled => proto::ContextMessageStatus {
+                variant: Some(proto::context_message_status::Variant::Canceled(
+                    proto::context_message_status::Canceled {},
+                )),
+            },
         }
     }
 }