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/user_shell_command.rs

@@ -3,8 +3,8 @@ use std::time::Duration;
 use codex_protocol::exec_output::ExecToolCallOutput;
 use codex_protocol::models::ResponseItem;
 
-use crate::codex::TurnContext;
 use crate::contextual_user_message::USER_SHELL_COMMAND_FRAGMENT;
+use crate::session::turn_context::TurnContext;
 use crate::tools::format_exec_output_str;
 
 fn format_duration_line(duration: Duration) -> String {