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/context_manager/history.rs

@@ -1,8 +1,8 @@
-use crate::codex::TurnContext;
 use crate::context_manager::normalize;
 use crate::event_mapping::has_non_contextual_dev_message_content;
 use crate::event_mapping::is_contextual_dev_message_content;
 use crate::event_mapping::is_contextual_user_message_content;
+use crate::session::turn_context::TurnContext;
 use base64::Engine;
 use base64::engine::general_purpose::STANDARD as BASE64_STANDARD;
 use codex_protocol::models::BaseInstructions;

codex-rs/core/src/context_manager/updates.rs

@@ -1,6 +1,6 @@
-use crate::codex::PreviousTurnSettings;
-use crate::codex::TurnContext;
 use crate::environment_context::EnvironmentContext;
+use crate::session::PreviousTurnSettings;
+use crate::session::turn_context::TurnContext;
 use crate::shell::Shell;
 use codex_execpolicy::Policy;
 use codex_features::Feature;