Lay the groundwork to support history in agent2 (#36483)

This pull request introduces title generation and history replaying. We
still need to wire up the rest of the history but this gets us very
close. I extracted a lot of this code from `agent2-history` because that
branch was starting to get long-lived and there were lots of changes
since we started.

Release Notes:

- N/A

crates/acp_thread/src/diff.rs

@@ -1,4 +1,3 @@
-use agent_client_protocol as acp;
 use anyhow::Result;
 use buffer_diff::{BufferDiff, BufferDiffSnapshot};
 use editor::{MultiBuffer, PathKey};
@@ -21,17 +20,13 @@ pub enum Diff {
 }
 
 impl Diff {
-    pub fn from_acp(
-        diff: acp::Diff,
+    pub fn finalized(
+        path: PathBuf,
+        old_text: Option<String>,
+        new_text: String,
         language_registry: Arc<LanguageRegistry>,
         cx: &mut Context<Self>,
     ) -> Self {
-        let acp::Diff {
-            path,
-            old_text,
-            new_text,
-        } = diff;
-
         let multibuffer = cx.new(|_cx| MultiBuffer::without_headers(Capability::ReadOnly));
 
         let new_buffer = cx.new(|cx| Buffer::local(new_text, cx));