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/state/session.rs

@@ -7,9 +7,9 @@ use std::collections::HashMap;
 use std::collections::HashSet;
 
 use crate::agent_identity::RegisteredAgentTask;
-use crate::codex::PreviousTurnSettings;
-use crate::codex::SessionConfiguration;
 use crate::context_manager::ContextManager;
+use crate::session::PreviousTurnSettings;
+use crate::session::session::SessionConfiguration;
 use crate::session_startup_prewarm::SessionStartupPrewarmHandle;
 use codex_protocol::protocol::RateLimitSnapshot;
 use codex_protocol::protocol::TokenUsage;