Allow attaching text threads as context (#29947)

Release Notes:

- N/A

---------

Co-authored-by: Michael Sloan <mgsloan@gmail.com>

crates/assistant_context_editor/src/context.rs

@@ -32,7 +32,7 @@ use serde::{Deserialize, Serialize};
 use smallvec::SmallVec;
 use std::{
     cmp::{Ordering, max},
-    fmt::Debug,
+    fmt::{Debug, Write as _},
     iter, mem,
     ops::Range,
     path::Path,
@@ -2539,6 +2539,26 @@ impl AssistantContext {
         Some(user_message)
     }
 
+    pub fn to_xml(&self, cx: &App) -> String {
+        let mut output = String::new();
+        let buffer = self.buffer.read(cx);
+        for message in self.messages(cx) {
+            if message.status != MessageStatus::Done {
+                continue;
+            }
+
+            writeln!(&mut output, "<{}>", message.role).unwrap();
+            for chunk in buffer.text_for_range(message.offset_range) {
+                output.push_str(chunk);
+            }
+            if !output.ends_with('\n') {
+                output.push('\n');
+            }
+            writeln!(&mut output, "</{}>", message.role).unwrap();
+        }
+        output
+    }
+
     pub fn to_completion_request(
         &self,
         request_type: RequestType,

crates/assistant_context_editor/src/context_store.rs

@@ -339,7 +339,11 @@ impl ContextStore {
         }
     }
 
-    pub fn contexts(&self) -> Vec<SavedContextMetadata> {
+    pub fn unordered_contexts(&self) -> impl Iterator<Item = &SavedContextMetadata> {
+        self.contexts_metadata.iter()
+    }
+
+    pub fn reverse_chronological_contexts(&self) -> Vec<SavedContextMetadata> {
         let mut contexts = self.contexts_metadata.iter().cloned().collect::<Vec<_>>();
         contexts.sort_unstable_by_key(|thread| std::cmp::Reverse(thread.mtime));
         contexts