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)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/2345).
* #2345
* #2329
* __->__ #2343
* #2340
* #2338

codex-rs/core/src/client.rs

@@ -56,7 +56,7 @@ struct Error {
     message: Option<String>,
 }
 
-#[derive(Clone)]
+#[derive(Debug, Clone)]
 pub struct ModelClient {
     config: Arc<Config>,
     auth: Option<CodexAuth>,