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/mcp_tool_call.rs

@@ -12,8 +12,6 @@ use tracing::error;
 
 use crate::arc_monitor::ArcMonitorOutcome;
 use crate::arc_monitor::monitor_action;
-use crate::codex::Session;
-use crate::codex::TurnContext;
 use crate::config::Config;
 use crate::config::edit::ConfigEdit;
 use crate::config::edit::ConfigEditsBuilder;
@@ -30,6 +28,8 @@ use crate::guardian::routes_approval_to_guardian;
 use crate::mcp_openai_file::rewrite_mcp_tool_arguments_for_openai_files;
 use crate::mcp_tool_approval_templates::RenderedMcpToolApprovalParam;
 use crate::mcp_tool_approval_templates::render_mcp_tool_approval_template;
+use crate::session::session::Session;
+use crate::session::turn_context::TurnContext;
 use codex_analytics::AppInvocation;
 use codex_analytics::InvocationType;
 use codex_analytics::build_track_events_context;