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/compact_remote.rs

@@ -2,9 +2,6 @@ use std::collections::HashSet;
 use std::sync::Arc;
 
 use crate::Prompt;
-use crate::codex::Session;
-use crate::codex::TurnContext;
-use crate::codex::built_tools;
 use crate::compact::CompactionAnalyticsAttempt;
 use crate::compact::InitialContextInjection;
 use crate::compact::compaction_status_from_result;
@@ -13,6 +10,9 @@ use crate::context_manager::ContextManager;
 use crate::context_manager::TotalTokenUsageBreakdown;
 use crate::context_manager::estimate_response_item_model_visible_bytes;
 use crate::context_manager::is_codex_generated_item;
+use crate::session::session::Session;
+use crate::session::turn::built_tools;
+use crate::session::turn_context::TurnContext;
 use codex_analytics::CompactionImplementation;
 use codex_analytics::CompactionPhase;
 use codex_analytics::CompactionReason;