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/environment_context.rs

@@ -1,5 +1,5 @@
-use crate::codex::TurnContext;
 use crate::contextual_user_message::ENVIRONMENT_CONTEXT_FRAGMENT;
+use crate::session::turn_context::TurnContext;
 use crate::shell::Shell;
 use codex_protocol::models::ResponseItem;
 use codex_protocol::protocol::TurnContextItem;