fix: Features should be immutable over the lifetime of a session/thread (#7540)

I noticed that `features: Features` was defined on `struct
SessionConfiguration`, which is commonly owned by `SessionState`, which
is in turn owned by `Session`.

Though I do not believe that `Features` should be allowed to be modified
over the course of a session (if the feature state is not invariant, it
makes it harder to reason about), which argues that it should live on
`Session` rather than `SessionState` or `SessionConfiguration`.

This PR moves `Features` to `Session` and updates all call sites. It
appears the only place we were mutating `Features` was:

- in tests
- the sub-agent config for a review task:


https://github.com/openai/codex/blob/3ef76ff29d5eed258fb6b8550e0e2b973d0dca21/codex-rs/core/src/tasks/review.rs#L86-L89

Note this change also means it is no longer an `async` call to check the
state of a feature, eliminating the possibility of a
[TOCTTOU](https://en.wikipedia.org/wiki/Time-of-check_to_time-of-use)
error between checking the state of a feature and acting on it:


https://github.com/openai/codex/blob/3ef76ff29d5eed258fb6b8550e0e2b973d0dca21/codex-rs/core/src/codex.rs#L1069-L1076

codex-rs/core/src/tasks/compact.rs

@@ -25,7 +25,7 @@ impl SessionTask for CompactTask {
         _cancellation_token: CancellationToken,
     ) -> Option<String> {
         let session = session.clone_session();
-        if crate::compact::should_use_remote_compact_task(&session).await {
+        if crate::compact::should_use_remote_compact_task(&session) {
             crate::compact_remote::run_remote_compact_task(session, ctx).await
         } else {
             crate::compact::run_compact_task(session, ctx, input).await