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/environment_context.rs

@@ -93,6 +93,25 @@ impl EnvironmentContext {
             && self.network_access == *network_access
             && self.writable_roots == *writable_roots
     }
+
+    pub fn diff(before: &TurnContext, after: &TurnContext) -> Self {
+        let cwd = if before.cwd != after.cwd {
+            Some(after.cwd.clone())
+        } else {
+            None
+        };
+        let approval_policy = if before.approval_policy != after.approval_policy {
+            Some(after.approval_policy)
+        } else {
+            None
+        };
+        let sandbox_policy = if before.sandbox_policy != after.sandbox_policy {
+            Some(after.sandbox_policy.clone())
+        } else {
+            None
+        };
+        EnvironmentContext::new(cwd, approval_policy, sandbox_policy, None)
+    }
 }
 
 impl From<&TurnContext> for EnvironmentContext {