assistant edit tool: Track read buffers and notify model of user edits (#26952)

When the model reads file, we'll track the version it read, and let it
know if the user makes edits to the buffer. This helps prevent edit
failures because it'll know to re-read the file before.

Release Notes:

- N/A

crates/assistant2/src/thread.rs

@@ -1,3 +1,4 @@
+use std::fmt::Write as _;
 use std::io::Write;
 use std::sync::Arc;
 
@@ -560,9 +561,39 @@ impl Thread {
             request.messages.push(context_message);
         }
 
+        self.attach_stale_files(&mut request.messages, cx);
+
         request
     }
 
+    fn attach_stale_files(&self, messages: &mut Vec<LanguageModelRequestMessage>, cx: &App) {
+        const STALE_FILES_HEADER: &str = "These files changed since last read:";
+
+        let mut stale_message = String::new();
+
+        for stale_file in self.action_log.read(cx).stale_buffers(cx) {
+            let Some(file) = stale_file.read(cx).file() else {
+                continue;
+            };
+
+            if stale_message.is_empty() {
+                write!(&mut stale_message, "{}", STALE_FILES_HEADER).ok();
+            }
+
+            writeln!(&mut stale_message, "- {}", file.path().display()).ok();
+        }
+
+        if !stale_message.is_empty() {
+            let context_message = LanguageModelRequestMessage {
+                role: Role::User,
+                content: vec![stale_message.into()],
+                cache: false,
+            };
+
+            messages.push(context_message);
+        }
+    }
+
     pub fn stream_completion(
         &mut self,
         request: LanguageModelRequest,