Create independent TurnContexts (#5308)

The goal of this change:
1. Unify user input and user turn implementation.
2. Have a single place where turn/session setting overrides are applied.
3. Have a single place where turn context is created.
4. Create TurnContext only for actual turn and have a separate structure
for current session settings (reuse ConfigureSession)

codex-rs/core/src/state/session.rs

@@ -2,14 +2,15 @@
 
 use codex_protocol::models::ResponseItem;
 
+use crate::codex::SessionConfiguration;
 use crate::conversation_history::ConversationHistory;
 use crate::protocol::RateLimitSnapshot;
 use crate::protocol::TokenUsage;
 use crate::protocol::TokenUsageInfo;
 
 /// Persistent, session-scoped state previously stored directly on `Session`.
-#[derive(Default)]
 pub(crate) struct SessionState {
+    pub(crate) session_configuration: SessionConfiguration,
     pub(crate) history: ConversationHistory,
     pub(crate) token_info: Option<TokenUsageInfo>,
     pub(crate) latest_rate_limits: Option<RateLimitSnapshot>,
@@ -17,10 +18,12 @@ pub(crate) struct SessionState {
 
 impl SessionState {
     /// Create a new session state mirroring previous `State::default()` semantics.
-    pub(crate) fn new() -> Self {
+    pub(crate) fn new(session_configuration: SessionConfiguration) -> Self {
         Self {
+            session_configuration,
             history: ConversationHistory::new(),
-            ..Default::default()
+            token_info: None,
+            latest_rate_limits: None,
         }
     }