[codex] Migrate thread turns list to thread store (#19280)

- migrate `thread/turns/list` to ThreadStore. Uses ThreadStore for most
data now but merges in the in-memory state from thread manager
- keep v2 `thread/list` pathless-store friendly by converting
`StoredThread` directly to API `Thread`
- add regression coverage for pathless store history/listing

codex-rs/config/src/config_toml.rs

@@ -427,7 +427,6 @@ pub enum ThreadStoreToml {
     Remote {
         endpoint: String,
     },
-    #[cfg(debug_assertions)]
     #[schemars(skip)]
     InMemory {
         id: String,