assistant2: Wire up context picker with inline assist (#22106)

This PR wire up the context picker with the inline assist.

UI is not finalized.

Release Notes:

- N/A

---------

Co-authored-by: Richard <richard@zed.dev>
Co-authored-by: Agus <agus@zed.dev>

crates/assistant2/src/context.rs

@@ -1,4 +1,5 @@
 use gpui::SharedString;
+use language_model::{LanguageModelRequestMessage, MessageContent};
 use serde::{Deserialize, Serialize};
 use util::post_inc;
 
@@ -26,3 +27,51 @@ pub enum ContextKind {
     FetchedUrl,
     Thread,
 }
+
+pub fn attach_context_to_message(
+    message: &mut LanguageModelRequestMessage,
+    context: impl IntoIterator<Item = Context>,
+) {
+    let mut file_context = String::new();
+    let mut fetch_context = String::new();
+    let mut thread_context = String::new();
+
+    for context in context.into_iter() {
+        match context.kind {
+            ContextKind::File => {
+                file_context.push_str(&context.text);
+                file_context.push('\n');
+            }
+            ContextKind::FetchedUrl => {
+                fetch_context.push_str(&context.name);
+                fetch_context.push('\n');
+                fetch_context.push_str(&context.text);
+                fetch_context.push('\n');
+            }
+            ContextKind::Thread => {
+                thread_context.push_str(&context.name);
+                thread_context.push('\n');
+                thread_context.push_str(&context.text);
+                thread_context.push('\n');
+            }
+        }
+    }
+
+    let mut context_text = String::new();
+    if !file_context.is_empty() {
+        context_text.push_str("The following files are available:\n");
+        context_text.push_str(&file_context);
+    }
+
+    if !fetch_context.is_empty() {
+        context_text.push_str("The following fetched results are available\n");
+        context_text.push_str(&fetch_context);
+    }
+
+    if !thread_context.is_empty() {
+        context_text.push_str("The following previous conversation threads are available\n");
+        context_text.push_str(&thread_context);
+    }
+
+    message.content.push(MessageContent::Text(context_text));
+}