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/unified_exec/mod.rs

@@ -34,9 +34,9 @@ use rand::Rng;
 use rand::rng;
 use tokio::sync::Mutex;
 
-use crate::codex::Session;
-use crate::codex::TurnContext;
 use crate::sandboxing::SandboxPermissions;
+use crate::session::session::Session;
+use crate::session::turn_context::TurnContext;
 
 mod async_watcher;
 mod errors;