TUI: enforce core boundary (#17399)

Problem: The TUI still depended on `codex-core` directly in a number of
places, and we had no enforcement from keeping this problem from getting
worse.

Solution: Route TUI core access through
`codex-app-server-client::legacy_core`, add CI enforcement for that
boundary, and re-export this legacy bridge inside the TUI as
`crate::legacy_core` so the remaining call sites stay readable. There is
no functional change in this PR — just changes to import targets.

Over time, we can whittle away at the remaining symbols in this legacy
namespace with the eventual goal of removing them all. In the meantime,
this linter rule will prevent us from inadvertently importing new
symbols from core.

codex-rs/tui/src/chatwidget/tests/helpers.rs

@@ -109,7 +109,7 @@ pub(super) fn snapshot(percent: f64) -> RateLimitSnapshot {
 }
 
 pub(super) fn test_session_telemetry(config: &Config, model: &str) -> SessionTelemetry {
-    let model_info = codex_core::test_support::construct_model_info_offline(model, config);
+    let model_info = crate::legacy_core::test_support::construct_model_info_offline(model, config);
     SessionTelemetry::new(
         ThreadId::new(),
         model,
@@ -131,7 +131,7 @@ pub(super) fn test_model_catalog(config: &Config) -> Arc<ModelCatalog> {
             .enabled(Feature::DefaultModeRequestUserInput),
     };
     Arc::new(ModelCatalog::new(
-        codex_core::test_support::all_model_presets().clone(),
+        crate::legacy_core::test_support::all_model_presets().clone(),
         collaboration_modes_config,
     ))
 }
@@ -148,9 +148,9 @@ pub(super) async fn make_chatwidget_manual(
     let app_event_tx = AppEventSender::new(tx_raw);
     let (op_tx, op_rx) = unbounded_channel::<Op>();
     let mut cfg = test_config().await;
-    let resolved_model = model_override
-        .map(str::to_owned)
-        .unwrap_or_else(|| codex_core::test_support::get_model_offline(cfg.model.as_deref()));
+    let resolved_model = model_override.map(str::to_owned).unwrap_or_else(|| {
+        crate::legacy_core::test_support::get_model_offline(cfg.model.as_deref())
+    });
     if let Some(model) = model_override {
         cfg.model = Some(model.to_string());
     }

codex-rs/tui/src/chatwidget/tests/plan_mode.rs

@@ -956,7 +956,7 @@ async fn submit_user_message_emits_structured_plugin_mentions_from_bindings() {
     });
     chat.set_feature_enabled(Feature::Plugins, /*enabled*/ true);
     chat.bottom_pane.set_plugin_mentions(Some(vec![
-        codex_core::plugins::PluginCapabilitySummary {
+        crate::legacy_core::plugins::PluginCapabilitySummary {
             config_name: "sample@test".to_string(),
             display_name: "Sample Plugin".to_string(),
             description: None,
@@ -1232,7 +1232,7 @@ async fn collaboration_modes_defaults_to_code_on_startup() {
         .build()
         .await
         .expect("config");
-    let resolved_model = codex_core::test_support::get_model_offline(cfg.model.as_deref());
+    let resolved_model = crate::legacy_core::test_support::get_model_offline(cfg.model.as_deref());
     let session_telemetry = test_session_telemetry(&cfg, resolved_model.as_str());
     let init = ChatWidgetInit {
         config: cfg.clone(),

codex-rs/tui/src/chatwidget/tests/popups_and_settings.rs

@@ -58,7 +58,7 @@ async fn experimental_mode_plan_is_ignored_on_startup() {
         .build()
         .await
         .expect("config");
-    let resolved_model = codex_core::test_support::get_model_offline(cfg.model.as_deref());
+    let resolved_model = crate::legacy_core::test_support::get_model_offline(cfg.model.as_deref());
     let session_telemetry = test_session_telemetry(&cfg, resolved_model.as_str());
     let init = ChatWidgetInit {
         config: cfg.clone(),

codex-rs/tui/src/chatwidget/tests/status_and_layout.rs

@@ -117,7 +117,7 @@ async fn helpers_are_available_and_do_not_panic() {
     let (tx_raw, _rx) = unbounded_channel::<AppEvent>();
     let tx = AppEventSender::new(tx_raw);
     let cfg = test_config().await;
-    let resolved_model = codex_core::test_support::get_model_offline(cfg.model.as_deref());
+    let resolved_model = crate::legacy_core::test_support::get_model_offline(cfg.model.as_deref());
     let session_telemetry = test_session_telemetry(&cfg, resolved_model.as_str());
     let init = ChatWidgetInit {
         config: cfg.clone(),