acp thread view: Always use editors for user messages (#36256)

This means the cursor will be at the position you clicked:


https://github.com/user-attachments/assets/0693950d-7513-4d90-88e2-55817df7213a


Release Notes:

- N/A

crates/acp_thread/src/acp_thread.rs

@@ -109,7 +109,7 @@ pub enum AgentThreadEntry {
 }
 
 impl AgentThreadEntry {
-    fn to_markdown(&self, cx: &App) -> String {
+    pub fn to_markdown(&self, cx: &App) -> String {
         match self {
             Self::UserMessage(message) => message.to_markdown(cx),
             Self::AssistantMessage(message) => message.to_markdown(cx),
@@ -117,6 +117,14 @@ impl AgentThreadEntry {
         }
     }
 
+    pub fn user_message(&self) -> Option<&UserMessage> {
+        if let AgentThreadEntry::UserMessage(message) = self {
+            Some(message)
+        } else {
+            None
+        }
+    }
+
     pub fn diffs(&self) -> impl Iterator<Item = &Entity<Diff>> {
         if let AgentThreadEntry::ToolCall(call) = self {
             itertools::Either::Left(call.diffs())