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/unified_exec/async_watcher.rs

@@ -8,9 +8,9 @@ use tokio::time::Sleep;
 
 use super::UnifiedExecContext;
 use super::process::UnifiedExecProcess;
-use crate::codex::Session;
-use crate::codex::TurnContext;
 use crate::exec::MAX_EXEC_OUTPUT_DELTAS_PER_CALL;
+use crate::session::session::Session;
+use crate::session::turn_context::TurnContext;
 use crate::tools::events::ToolEmitter;
 use crate::tools::events::ToolEventCtx;
 use crate::tools::events::ToolEventFailure;

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

@@ -34,9 +34,9 @@ use rand::Rng;
 use rand::rng;
 use tokio::sync::Mutex;
 
-use crate::codex::Session;
-use crate::codex::TurnContext;
 use crate::sandboxing::SandboxPermissions;
+use crate::session::session::Session;
+use crate::session::turn_context::TurnContext;
 
 mod async_watcher;
 mod errors;

codex-rs/core/src/unified_exec/mod_tests.rs

@@ -1,11 +1,11 @@
 use super::head_tail_buffer::HeadTailBuffer;
 use super::*;
-use crate::codex::Session;
-use crate::codex::TurnContext;
-use crate::codex::make_session_and_context;
 use crate::exec::ExecCapturePolicy;
 use crate::exec::ExecExpiration;
 use crate::sandboxing::ExecRequest;
+use crate::session::session::Session;
+use crate::session::tests::make_session_and_context;
+use crate::session::turn_context::TurnContext;
 use crate::tools::context::ExecCommandToolOutput;
 use crate::unified_exec::WriteStdinRequest;
 use crate::unified_exec::process::OutputHandles;