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/compact_tests.rs

@@ -7,7 +7,7 @@ async fn process_compacted_history_with_test_session(
     compacted_history: Vec<ResponseItem>,
     previous_turn_settings: Option<&PreviousTurnSettings>,
 ) -> (Vec<ResponseItem>, Vec<ResponseItem>) {
-    let (session, turn_context) = crate::codex::make_session_and_context().await;
+    let (session, turn_context) = crate::session::tests::make_session_and_context().await;
     session
         .set_previous_turn_settings(previous_turn_settings.cloned())
         .await;