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/realtime_conversation.rs

@@ -1,7 +1,7 @@
 use crate::client::ModelClient;
-use crate::codex::Session;
 use crate::realtime_context::build_realtime_startup_context;
 use crate::realtime_prompt::prepare_realtime_backend_prompt;
+use crate::session::session::Session;
 use anyhow::Context;
 use async_channel::Receiver;
 use async_channel::RecvError;