Centralize thread environment selection state (#38423)

## What changed

- Make `ThreadEnvironments` the source of truth for live environment selections and use it when building configuration snapshots, permission profiles, per-turn configuration, and MCP refresh inputs.
- Keep settings previews side-effect free while applying accepted environment updates consistently to subsequent turns.
- Move `EnvironmentConfig` into `codex-protocol` so environment ownership no longer depends on a core-only type.

## Testing

- Cover environment previews and snapshots, accepted and rejected steer updates, and active-turn environment stability.

GitOrigin-RevId: 7bcf8fb5df1215f720e4e9c419541a51a871aa5e

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

@@ -127,7 +127,6 @@ use codex_protocol::protocol::TurnAbortReason;
 use codex_protocol::protocol::TurnContextItem;
 use codex_protocol::protocol::TurnContextNetworkItem;
 use codex_protocol::protocol::TurnEnvironmentSelection;
-use codex_protocol::protocol::TurnEnvironmentSelections;
 use codex_protocol::protocol::W3cTraceContext;
 use codex_protocol::protocol::WorldStateItem;
 use codex_protocol::request_permissions::PermissionGrantScope;
@@ -697,10 +696,7 @@ impl Session {
             approvals_reviewer: config.approvals_reviewer,
             permission_profile_state: session_permission_profile_state_from_config(&config)?,
             windows_sandbox_level: WindowsSandboxLevel::from_config(&config),
-            environments: TurnEnvironmentSelections::new(
-                config.cwd.clone(),
-                environment_selections,
-            ),
+            legacy_fallback_cwd: config.cwd.clone(),
             codex_home: config.codex_home.clone(),
             thread_name: None,
             original_config_do_not_use: Arc::clone(&config),
@@ -718,7 +714,7 @@ impl Session {
             user_shell_override,
         };
         session_configuration
-            .validate_auto_review_requirement()
+            .validate_auto_review_requirement(&environment_selections)
             .map_err(|err| CodexErr::InvalidRequest(err.to_string()))?;
 
         // Generate a unique ID for the lifetime of this session.
@@ -727,6 +723,7 @@ impl Session {
 
         let session = Box::pin(Session::new(
             session_configuration,
+            &environment_selections,
             config.clone(),
             user_instructions,
             installation_id,
@@ -1570,7 +1567,8 @@ impl Session {
         let notify_config_contributors = !self.services.extensions.config_contributors().is_empty();
         let (previous_config, new_config, permission_profile_changed, mcp_inputs_changed) = {
             let mut state = self.state.lock().await;
-            let updated = match state.session_configuration.apply(&updates) {
+            let updated = match self.apply_session_settings(&state.session_configuration, &updates)
+            {
                 Ok(updated) => updated,
                 Err(err) => {
                     warn!("rejected session settings update: {err}");
@@ -1579,25 +1577,26 @@ impl Session {
             };
 
             let previous_config = notify_config_contributors
-                .then(|| Self::build_effective_session_config(&state.session_configuration));
-            let new_config =
-                notify_config_contributors.then(|| Self::build_effective_session_config(&updated));
+                .then(|| self.build_effective_session_config(&state.session_configuration));
             let previous_permission_profile = state.session_configuration.permission_profile();
             let updated_permission_profile = updated.permission_profile();
             let permission_profile_changed =
                 previous_permission_profile != updated_permission_profile;
-            let mcp_inputs_changed = state.session_configuration.mcp_inputs_differ(&updated);
+            let mcp_inputs_changed =
+                self.mcp_inputs_differ(&state.session_configuration, &updated, &updates);
             let environment_config = updated.turn_environment_config();
-            if updates.environments.is_some() {
+            if let Some(environments) = &updates.environments {
                 self.services
                     .turn_environments
-                    .update_selections(updated.environment_selections(), &environment_config);
+                    .update_selections(&environments.environments, &environment_config);
             } else if state.session_configuration.turn_environment_config() != environment_config {
                 self.services
                     .turn_environments
                     .update_environment_configs(&environment_config);
             }
             state.session_configuration = updated;
+            let new_config = notify_config_contributors
+                .then(|| self.build_effective_session_config(&state.session_configuration));
             if mcp_inputs_changed {
                 self.mark_mcp_runtime_dirty();
             }
@@ -1624,15 +1623,19 @@ impl Session {
         updates: &SessionSettingsUpdate,
     ) -> ConstraintResult<ThreadConfigSnapshot> {
         let state = self.state.lock().await;
-        state
-            .session_configuration
-            .apply(updates)
-            .map(|configuration| configuration.thread_config_snapshot())
+        let configuration = self.apply_session_settings(&state.session_configuration, updates)?;
+        let environments = updates.environments.as_ref().map_or_else(
+            || self.services.turn_environments.selections(),
+            |environments| environments.environments.clone(),
+        );
+        Ok(configuration.thread_config_snapshot(environments))
     }
 
     pub(crate) async fn thread_config_snapshot(&self) -> ThreadConfigSnapshot {
         let state = self.state.lock().await;
-        state.session_configuration.thread_config_snapshot()
+        state
+            .session_configuration
+            .thread_config_snapshot(self.services.turn_environments.selections())
     }
 
     pub(crate) async fn set_app_server_client_info(
@@ -1662,14 +1665,6 @@ impl Session {
             .map_or_else(Vec::new, |instructions| instructions.sources().collect())
     }
 
-    pub(crate) async fn thread_environment_selections(&self) -> Vec<TurnEnvironmentSelection> {
-        let state = self.state.lock().await;
-        state
-            .session_configuration
-            .environment_selections()
-            .to_vec()
-    }
-
     pub(crate) async fn set_session_startup_prewarm(
         &self,
         startup_prewarm: SessionStartupPrewarmHandle,
@@ -1708,7 +1703,7 @@ impl Session {
         let (previous_config, new_config, config) = {
             let mut state = self.state.lock().await;
             let previous_config = notify_config_contributors
-                .then(|| Self::build_effective_session_config(&state.session_configuration));
+                .then(|| self.build_effective_session_config(&state.session_configuration));
             let mut config = (*state.session_configuration.original_config_do_not_use).clone();
             config.config_layer_stack = config
                 .config_layer_stack
@@ -1727,7 +1722,7 @@ impl Session {
             state.session_configuration.original_config_do_not_use = Arc::clone(&config);
             self.mark_mcp_runtime_dirty();
             let new_config = notify_config_contributors
-                .then(|| Self::build_effective_session_config(&state.session_configuration));
+                .then(|| self.build_effective_session_config(&state.session_configuration));
             (previous_config, new_config, config)
         };
         self.emit_config_changed_contributors(previous_config.as_ref(), new_config.as_ref());