fix: Features should be immutable over the lifetime of a session/thread

[bolinfest]

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:

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

Lines 86 to 89 in 3ef76ff

	sub_agent_config
	.features
	.disable(crate::features::Feature::WebSearchRequest)
	.disable(crate::features::Feature::ViewImageTool);

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 error between checking the state of a feature and acting on it:

codex/codex-rs/core/src/codex.rs

Lines 1069 to 1076 in 3ef76ff

	pub async fn enabled(&self, feature: Feature) -> bool {
	self.state
	.lock()
	.await
	.session_configuration
	.features
	.enabled(feature)
	}