feat: introduce TurnContext (#2343)

This PR introduces `TurnContext`, which is designed to hold a set of
fields that should be constant for a turn of a conversation. Note that
the fields of `TurnContext` were previously governed by `Session`.

Ultimately, we want to enable users to change these values between turns
(changing model, approval policy, etc.), though in the current
implementation, the `TurnContext` is constant for the entire
conversation.

---
[//]: # (BEGIN SAPLING FOOTER)
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with [ReviewStack](https://reviewstack.dev/openai/codex/pull/2345).
* #2345
* #2329
* __->__ #2343
* #2340
* #2338

codex-rs/core/src/apply_patch.rs

@@ -1,4 +1,5 @@
 use crate::codex::Session;
+use crate::codex::TurnContext;
 use crate::models::FunctionCallOutputPayload;
 use crate::models::ResponseInputItem;
 use crate::protocol::FileChange;
@@ -40,15 +41,16 @@ impl From<ResponseInputItem> for InternalApplyPatchInvocation {
 
 pub(crate) async fn apply_patch(
     sess: &Session,
+    turn_context: &TurnContext,
     sub_id: &str,
     call_id: &str,
     action: ApplyPatchAction,
 ) -> InternalApplyPatchInvocation {
     match assess_patch_safety(
         &action,
-        sess.get_approval_policy(),
-        sess.get_sandbox_policy(),
-        sess.get_cwd(),
+        turn_context.approval_policy,
+        &turn_context.sandbox_policy,
+        &turn_context.cwd,
     ) {
         SafetyCheck::AutoApprove { .. } => {
             InternalApplyPatchInvocation::DelegateToExec(ApplyPatchExec {