assistant2: Add support for editing the last message sent by the user (#26037)

https://github.com/user-attachments/assets/df46632b-dfeb-4991-ab2e-86829b72be9b

Closes #ISSUE

Release Notes:

- N/A

crates/assistant2/src/thread.rs

@@ -99,7 +99,13 @@ impl Thread {
         tools: Arc<ToolWorkingSet>,
         _cx: &mut Context<Self>,
     ) -> Self {
-        let next_message_id = MessageId(saved.messages.len());
+        let next_message_id = MessageId(
+            saved
+                .messages
+                .last()
+                .map(|message| message.id.0 + 1)
+                .unwrap_or(0),
+        );
         let tool_use = ToolUseState::from_saved_messages(&saved.messages);
 
         Self {
@@ -229,6 +235,34 @@ impl Thread {
         id
     }
 
+    pub fn edit_message(
+        &mut self,
+        id: MessageId,
+        new_role: Role,
+        new_text: String,
+        cx: &mut Context<Self>,
+    ) -> bool {
+        let Some(message) = self.messages.iter_mut().find(|message| message.id == id) else {
+            return false;
+        };
+        message.role = new_role;
+        message.text = new_text;
+        self.touch_updated_at();
+        cx.emit(ThreadEvent::MessageEdited(id));
+        true
+    }
+
+    pub fn delete_message(&mut self, id: MessageId, cx: &mut Context<Self>) -> bool {
+        let Some(index) = self.messages.iter().position(|message| message.id == id) else {
+            return false;
+        };
+        self.messages.remove(index);
+        self.context_by_message.remove(&id);
+        self.touch_updated_at();
+        cx.emit(ThreadEvent::MessageDeleted(id));
+        true
+    }
+
     /// Returns the representation of this [`Thread`] in a textual form.
     ///
     /// This is the representation we use when attaching a thread as context to another thread.
@@ -567,6 +601,8 @@ pub enum ThreadEvent {
     StreamedCompletion,
     StreamedAssistantText(MessageId, String),
     MessageAdded(MessageId),
+    MessageEdited(MessageId),
+    MessageDeleted(MessageId),
     SummaryChanged,
     UsePendingTools,
     ToolFinished {