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/state/session.rs

@@ -7,9 +7,9 @@ use std::collections::HashMap;
 use std::collections::HashSet;
 
 use crate::agent_identity::RegisteredAgentTask;
-use crate::codex::PreviousTurnSettings;
-use crate::codex::SessionConfiguration;
 use crate::context_manager::ContextManager;
+use crate::session::PreviousTurnSettings;
+use crate::session::session::SessionConfiguration;
 use crate::session_startup_prewarm::SessionStartupPrewarmHandle;
 use codex_protocol::protocol::RateLimitSnapshot;
 use codex_protocol::protocol::TokenUsage;

codex-rs/core/src/state/session_tests.rs

@@ -1,6 +1,6 @@
 use super::*;
 use crate::agent_identity::RegisteredAgentTask;
-use crate::codex::make_session_configuration_for_tests;
+use crate::session::tests::make_session_configuration_for_tests;
 use codex_protocol::protocol::CreditsSnapshot;
 use codex_protocol::protocol::RateLimitWindow;
 use pretty_assertions::assert_eq;

codex-rs/core/src/state/turn.rs

@@ -17,7 +17,7 @@ use codex_rmcp_client::ElicitationResponse;
 use rmcp::model::RequestId;
 use tokio::sync::oneshot;
 
-use crate::codex::TurnContext;
+use crate::session::turn_context::TurnContext;
 use crate::tasks::AnySessionTask;
 use codex_protocol::models::PermissionProfile;
 use codex_protocol::protocol::ReviewDecision;