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/code_mode/execute_handler.rs

@@ -15,8 +15,8 @@ pub struct CodeModeExecuteHandler;
 impl CodeModeExecuteHandler {
     async fn execute(
         &self,
-        session: std::sync::Arc<crate::codex::Session>,
-        turn: std::sync::Arc<crate::codex::TurnContext>,
+        session: std::sync::Arc<crate::session::session::Session>,
+        turn: std::sync::Arc<crate::session::turn_context::TurnContext>,
         call_id: String,
         code: String,
     ) -> Result<FunctionToolOutput, FunctionCallError> {

codex-rs/core/src/tools/code_mode/mod.rs

@@ -15,11 +15,11 @@ use codex_protocol::models::ResponseInputItem;
 use serde_json::Value as JsonValue;
 use tokio_util::sync::CancellationToken;
 
-use crate::codex::Session;
-use crate::codex::TurnContext;
 use crate::function_tool::FunctionCallError;
 use crate::original_image_detail::can_request_original_image_detail;
 use crate::original_image_detail::sanitize_original_image_detail as sanitize_image_detail_items;
+use crate::session::session::Session;
+use crate::session::turn_context::TurnContext;
 use crate::tools::ToolRouter;
 use crate::tools::context::FunctionToolOutput;
 use crate::tools::context::SharedTurnDiffTracker;