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_context.rs

@@ -1,6 +1,6 @@
-use crate::codex::Session;
 use crate::compact::content_items_to_text;
 use crate::event_mapping::is_contextual_user_message_content;
+use crate::session::session::Session;
 use chrono::Utc;
 use codex_exec_server::LOCAL_FS;
 use codex_git_utils::resolve_root_git_project_for_trust;