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/mention.rs

@@ -2,6 +2,7 @@ use agent::ThreadId;
 use anyhow::{Context as _, Result, bail};
 use file_icons::FileIcons;
 use prompt_store::{PromptId, UserPromptId};
+use serde::{Deserialize, Serialize};
 use std::{
     fmt,
     ops::Range,
@@ -11,7 +12,7 @@ use std::{
 use ui::{App, IconName, SharedString};
 use url::Url;
 
-#[derive(Clone, Debug, PartialEq, Eq)]
+#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
 pub enum MentionUri {
     File {
         abs_path: PathBuf,