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:


3ef76ff29d/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:


3ef76ff29d/codex-rs/core/src/codex.rs (L1069-L1076)

codex-rs/core/src/compact.rs

@@ -32,13 +32,13 @@ pub const SUMMARIZATION_PROMPT: &str = include_str!("../templates/compact/prompt
 pub const SUMMARY_PREFIX: &str = include_str!("../templates/compact/summary_prefix.md");
 const COMPACT_USER_MESSAGE_MAX_TOKENS: usize = 20_000;
 
-pub(crate) async fn should_use_remote_compact_task(session: &Session) -> bool {
+pub(crate) fn should_use_remote_compact_task(session: &Session) -> bool {
     session
         .services
         .auth_manager
         .auth()
         .is_some_and(|auth| auth.mode == AuthMode::ChatGPT)
-        && session.enabled(Feature::RemoteCompaction).await
+        && session.enabled(Feature::RemoteCompaction)
 }
 
 pub(crate) async fn run_inline_auto_compact_task(