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/compact.rs

@@ -2,7 +2,7 @@ use std::sync::Arc;
 
 use super::SessionTask;
 use super::SessionTaskContext;
-use crate::codex::TurnContext;
+use crate::session::turn_context::TurnContext;
 use crate::state::TaskKind;
 use codex_protocol::user_input::UserInput;
 use tokio_util::sync::CancellationToken;

codex-rs/core/src/tasks/ghost_snapshot.rs

@@ -1,4 +1,4 @@
-use crate::codex::TurnContext;
+use crate::session::turn_context::TurnContext;
 use crate::state::TaskKind;
 use crate::tasks::SessionTask;
 use crate::tasks::SessionTaskContext;

codex-rs/core/src/tasks/mod.rs

@@ -19,14 +19,14 @@ use tracing::info_span;
 use tracing::trace;
 use tracing::warn;
 
-use crate::codex::Session;
-use crate::codex::TurnContext;
 use crate::contextual_user_message::TURN_ABORTED_CLOSE_TAG;
 use crate::contextual_user_message::TURN_ABORTED_OPEN_TAG;
 use crate::hook_runtime::PendingInputHookDisposition;
 use crate::hook_runtime::inspect_pending_input;
 use crate::hook_runtime::record_additional_contexts;
 use crate::hook_runtime::record_pending_input;
+use crate::session::session::Session;
+use crate::session::turn_context::TurnContext;
 use crate::state::ActiveTurn;
 use crate::state::RunningTask;
 use crate::state::TaskKind;

codex-rs/core/src/tasks/regular.rs

@@ -2,8 +2,8 @@ use std::sync::Arc;
 
 use tokio_util::sync::CancellationToken;
 
-use crate::codex::TurnContext;
-use crate::codex::run_turn;
+use crate::session::turn::run_turn;
+use crate::session::turn_context::TurnContext;
 use crate::session_startup_prewarm::SessionStartupPrewarmResolution;
 use crate::state::TaskKind;
 use codex_protocol::protocol::EventMsg;

codex-rs/core/src/tasks/review.rs

@@ -17,12 +17,12 @@ use codex_protocol::protocol::SubAgentSource;
 use codex_utils_template::Template;
 use tokio_util::sync::CancellationToken;
 
-use crate::codex::Session;
-use crate::codex::TurnContext;
 use crate::codex_delegate::run_codex_thread_one_shot;
 use crate::config::Constrained;
 use crate::review_format::format_review_findings_block;
 use crate::review_format::render_review_output_text;
+use crate::session::session::Session;
+use crate::session::turn_context::TurnContext;
 use crate::state::TaskKind;
 use codex_features::Feature;
 use codex_protocol::user_input::UserInput;

codex-rs/core/src/tasks/undo.rs

@@ -1,6 +1,6 @@
 use std::sync::Arc;
 
-use crate::codex::TurnContext;
+use crate::session::turn_context::TurnContext;
 use crate::state::TaskKind;
 use crate::tasks::SessionTask;
 use crate::tasks::SessionTaskContext;

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;