chore: upgrade to Rust 1.90 (#4124)

Inspired by Dependabot's attempt to do this:
https://github.com/openai/codex/pull/4029

The new version of Clippy found some unused structs that are removed in
this PR.

Though nothing stood out to me in the Release Notes in terms of things
we should start to take advantage of:
https://blog.rust-lang.org/2025/09/18/Rust-1.90.0/.

codex-rs/core/src/rollout/recorder.rs

@@ -7,8 +7,6 @@ use std::path::Path;
 use std::path::PathBuf;
 
 use codex_protocol::mcp_protocol::ConversationId;
-use serde::Deserialize;
-use serde::Serialize;
 use serde_json::Value;
 use time::OffsetDateTime;
 use time::format_description::FormatItem;
@@ -28,7 +26,6 @@ use super::policy::is_persisted_response_item;
 use crate::config::Config;
 use crate::default_client::ORIGINATOR;
 use crate::git_info::collect_git_info;
-use codex_protocol::models::ResponseItem;
 use codex_protocol::protocol::InitialHistory;
 use codex_protocol::protocol::ResumedHistory;
 use codex_protocol::protocol::RolloutItem;
@@ -36,19 +33,6 @@ use codex_protocol::protocol::RolloutLine;
 use codex_protocol::protocol::SessionMeta;
 use codex_protocol::protocol::SessionMetaLine;
 
-#[derive(Serialize, Deserialize, Default, Clone)]
-pub struct SessionStateSnapshot {}
-
-#[derive(Serialize, Deserialize, Default, Clone)]
-pub struct SavedSession {
-    pub session: SessionMeta,
-    #[serde(default)]
-    pub items: Vec<ResponseItem>,
-    #[serde(default)]
-    pub state: SessionStateSnapshot,
-    pub session_id: ConversationId,
-}
-
 /// Records all [`ResponseItem`]s for a session and flushes them to disk after
 /// every update.
 ///