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/tasks/user_shell.rs

@@ -8,12 +8,12 @@ use tokio_util::sync::CancellationToken;
 use tracing::error;
 use uuid::Uuid;
 
-use crate::codex::TurnContext;
 use crate::exec::ExecCapturePolicy;
 use crate::exec::StdoutStream;
 use crate::exec::execute_exec_request;
 use crate::exec_env::create_env;
 use crate::sandboxing::ExecRequest;
+use crate::session::turn_context::TurnContext;
 use crate::state::TaskKind;
 use crate::tools::format_exec_output_str;
 use crate::tools::runtimes::maybe_wrap_shell_lc_with_snapshot;
@@ -32,7 +32,7 @@ use codex_shell_command::parse_command::parse_command;
 
 use super::SessionTask;
 use super::SessionTaskContext;
-use crate::codex::Session;
+use crate::session::session::Session;
 use codex_protocol::models::ResponseInputItem;
 use codex_protocol::models::ResponseItem;
 use codex_protocol::permissions::FileSystemSandboxPolicy;