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/codex_delegate.rs

@@ -30,13 +30,6 @@ use tokio::sync::oneshot;
 use tokio::time::timeout;
 use tokio_util::sync::CancellationToken;
 
-use crate::codex::Codex;
-use crate::codex::CodexSpawnArgs;
-use crate::codex::CodexSpawnOk;
-use crate::codex::SUBMISSION_CHANNEL_CAPACITY;
-use crate::codex::Session;
-use crate::codex::TurnContext;
-use crate::codex::emit_subagent_session_started;
 use crate::config::Config;
 use crate::guardian::GuardianApprovalRequest;
 use crate::guardian::new_guardian_review_id;
@@ -48,13 +41,20 @@ use crate::mcp_tool_call::MCP_TOOL_APPROVAL_DECLINE_SYNTHETIC;
 use crate::mcp_tool_call::build_guardian_mcp_tool_review_request;
 use crate::mcp_tool_call::is_mcp_tool_approval_question_id;
 use crate::mcp_tool_call::lookup_mcp_tool_metadata;
+use crate::session::Codex;
+use crate::session::CodexSpawnArgs;
+use crate::session::CodexSpawnOk;
+use crate::session::SUBMISSION_CHANNEL_CAPACITY;
+use crate::session::emit_subagent_session_started;
+use crate::session::session::Session;
+use crate::session::turn_context::TurnContext;
 use codex_login::AuthManager;
 use codex_models_manager::manager::ModelsManager;
 use codex_protocol::error::CodexErr;
 use codex_protocol::protocol::InitialHistory;
 
 #[cfg(test)]
-use crate::codex::completed_session_loop_termination;
+use crate::session::completed_session_loop_termination;
 
 /// Start an interactive sub-Codex thread and return IO channels.
 ///