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/acp_thread/Cargo.toml

@@ -13,7 +13,7 @@ path = "src/acp_thread.rs"
 doctest = false
 
 [features]
-test-support = ["gpui/test-support", "project/test-support"]
+test-support = ["gpui/test-support", "project/test-support", "dep:parking_lot"]
 
 [dependencies]
 action_log.workspace = true
@@ -29,6 +29,7 @@ gpui.workspace = true
 itertools.workspace = true
 language.workspace = true
 markdown.workspace = true
+parking_lot = { workspace = true, optional = true }
 project.workspace = true
 prompt_store.workspace = true
 serde.workspace = true