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/apply_patch.rs

@@ -1,7 +1,7 @@
-use crate::codex::TurnContext;
 use crate::function_tool::FunctionCallError;
 use crate::safety::SafetyCheck;
 use crate::safety::assess_patch_safety;
+use crate::session::turn_context::TurnContext;
 use crate::tools::sandboxing::ExecApprovalRequirement;
 use codex_apply_patch::ApplyPatchAction;
 use codex_apply_patch::ApplyPatchFileChange;