agent2: Initial infra for checkpoints and message editing (#36120)

Release Notes:

- N/A

---------

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

crates/acp_thread/Cargo.toml

@@ -36,6 +36,7 @@ terminal.workspace = true
 ui.workspace = true
 url.workspace = true
 util.workspace = true
+uuid.workspace = true
 watch.workspace = true
 workspace-hack.workspace = true