agent: Snapshot context in user message instead of recreating it (#27967)

This makes context essentially work the same way as `read-file`,
increasing the likelihood of cache hits.

Just like with `read-file`, we'll notify the model when the user makes
an edit to one of the tracked files. In the future, we want to send a
diff instead of just a list of files, but that's an orthogonal change.


Release Notes:
- agent: Improved caching of files in context

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>

crates/agent/src/context.rs

@@ -146,11 +146,11 @@ pub struct ContextSymbolId {
     pub range: Range<Anchor>,
 }
 
-pub fn attach_context_to_message<'a>(
-    message: &mut LanguageModelRequestMessage,
+/// Formats a collection of contexts into a string representation
+pub fn format_context_as_string<'a>(
     contexts: impl Iterator<Item = &'a AssistantContext>,
     cx: &App,
-) {
+) -> Option<String> {
     let mut file_context = Vec::new();
     let mut directory_context = Vec::new();
     let mut symbol_context = Vec::new();
@@ -167,64 +167,78 @@ pub fn attach_context_to_message<'a>(
         }
     }
 
-    let mut context_chunks = Vec::new();
+    if file_context.is_empty()
+        && directory_context.is_empty()
+        && symbol_context.is_empty()
+        && fetch_context.is_empty()
+        && thread_context.is_empty()
+    {
+        return None;
+    }
+
+    let mut result = String::new();
+    result.push_str("\n<context>\n\
+        The following items were attached by the user. You don't need to use other tools to read them.\n\n");
 
     if !file_context.is_empty() {
-        context_chunks.push("<files>\n");
+        result.push_str("<files>\n");
         for context in file_context {
-            context_chunks.push(&context.context_buffer.text);
+            result.push_str(&context.context_buffer.text);
         }
-        context_chunks.push("\n</files>\n");
+        result.push_str("</files>\n");
     }
 
     if !directory_context.is_empty() {
-        context_chunks.push("<directories>\n");
+        result.push_str("<directories>\n");
         for context in directory_context {
             for context_buffer in &context.context_buffers {
-                context_chunks.push(&context_buffer.text);
+                result.push_str(&context_buffer.text);
             }
         }
-        context_chunks.push("\n</directories>\n");
+        result.push_str("</directories>\n");
     }
 
     if !symbol_context.is_empty() {
-        context_chunks.push("<symbols>\n");
+        result.push_str("<symbols>\n");
         for context in symbol_context {
-            context_chunks.push(&context.context_symbol.text);
+            result.push_str(&context.context_symbol.text);
+            result.push('\n');
         }
-        context_chunks.push("\n</symbols>\n");
+        result.push_str("</symbols>\n");
     }
 
     if !fetch_context.is_empty() {
-        context_chunks.push("<fetched_urls>\n");
+        result.push_str("<fetched_urls>\n");
         for context in &fetch_context {
-            context_chunks.push(&context.url);
-            context_chunks.push(&context.text);
+            result.push_str(&context.url);
+            result.push('\n');
+            result.push_str(&context.text);
+            result.push('\n');
         }
-        context_chunks.push("\n</fetched_urls>\n");
+        result.push_str("</fetched_urls>\n");
     }
 
-    // Need to own the SharedString for summary so that it can be referenced.
-    let mut thread_context_chunks = Vec::new();
     if !thread_context.is_empty() {
-        context_chunks.push("<conversation_threads>\n");
+        result.push_str("<conversation_threads>\n");
         for context in &thread_context {
-            thread_context_chunks.push(context.summary(cx));
-            thread_context_chunks.push(context.text.clone());
+            result.push_str(&context.summary(cx));
+            result.push('\n');
+            result.push_str(&context.text);
+            result.push('\n');
         }
-        context_chunks.push("\n</conversation_threads>\n");
+        result.push_str("</conversation_threads>\n");
     }
 
-    for chunk in &thread_context_chunks {
-        context_chunks.push(chunk);
-    }
+    result.push_str("</context>\n");
+    Some(result)
+}
 
-    if !context_chunks.is_empty() {
-        message.content.push(
-            "\n<context>\n\
-                The following items were attached by the user. You don't need to use other tools to read them.\n\n".into(),
-        );
-        message.content.push(context_chunks.join("\n").into());
-        message.content.push("\n</context>\n".into());
+pub fn attach_context_to_message<'a>(
+    message: &mut LanguageModelRequestMessage,
+    contexts: impl Iterator<Item = &'a AssistantContext>,
+    cx: &App,
+) {
+    if let Some(context_string) = format_context_as_string(contexts, cx) {
+        message.content.push(context_string.into());
     }
 }