acp: Clean up entry views on rewind (#36197)

We were leaking diffs and terminals on rewind, we'll now clean them up.
This PR also introduces a refactor of how we mantain the entry view
state to use a `Vec` that's kept in sync with the thread entries.

Release Notes:

- N/A

crates/agent_ui/Cargo.toml

@@ -103,6 +103,7 @@ workspace.workspace = true
 zed_actions.workspace = true
 
 [dev-dependencies]
+acp_thread = { workspace = true, features = ["test-support"] }
 agent = { workspace = true, features = ["test-support"] }
 assistant_context = { workspace = true, features = ["test-support"] }
 assistant_tools.workspace = true