Move codex module under session (#18249)

## Summary
- rename the core codex module root to session/mod.rs without using
#[path]
- move the codex module directory and tests under core/src/session
- remove session/mod.rs reexports so call sites use explicit child
module paths

## Testing
- cargo test -p codex-core --lib
- cargo check -p codex-core --tests
- just fmt
- just fix -p codex-core
- git diff --check

codex-rs/core/src/turn_timing.rs

@@ -10,7 +10,7 @@ use codex_protocol::models::ResponseItem;
 use tokio::sync::Mutex;
 
 use crate::ResponseEvent;
-use crate::codex::TurnContext;
+use crate::session::turn_context::TurnContext;
 use crate::stream_events_utils::raw_assistant_output_text_from_item;
 
 pub(crate) async fn record_turn_ttft_metric(turn_context: &TurnContext, event: &ResponseEvent) {