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/lib.rs

@@ -11,11 +11,11 @@ mod apps;
 mod arc_monitor;
 mod client;
 mod client_common;
-pub(crate) mod codex;
 mod realtime_context;
 mod realtime_conversation;
 mod realtime_prompt;
-pub use codex::SteerInputError;
+pub(crate) mod session;
+pub use session::SteerInputError;
 mod codex_thread;
 mod compact_remote;
 pub use codex_thread::CodexThread;