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/tools/runtimes/shell/unix_escalation.rs

@@ -300,8 +300,8 @@ pub(crate) async fn prepare_unified_exec_zsh_fork(
 
 struct CoreShellActionProvider {
     policy: Arc<RwLock<Policy>>,
-    session: Arc<crate::codex::Session>,
-    turn: Arc<crate::codex::TurnContext>,
+    session: Arc<crate::session::session::Session>,
+    turn: Arc<crate::session::turn_context::TurnContext>,
     call_id: String,
     tool_name: GuardianCommandSource,
     approval_policy: AskForApproval,

codex-rs/core/src/tools/runtimes/shell/unix_escalation_tests.rs

@@ -6,9 +6,9 @@ use super::evaluate_intercepted_exec_policy;
 use super::extract_shell_script;
 use super::join_program_and_argv;
 use super::map_exec_result;
-use crate::codex::make_session_and_context;
 use crate::config::Constrained;
 use crate::sandboxing::SandboxPermissions;
+use crate::session::tests::make_session_and_context;
 use anyhow::Context;
 use codex_execpolicy::Decision;
 use codex_execpolicy::Evaluation;