Move TUI app tests to modules they cover (#18799)

## Summary

The TUI app refactor in #18753 moved the old `app.rs` tests into a
single `app/tests.rs` file. That kept the split mechanically simple, but
it left several focused unit tests far from the modules they exercise.

This PR is a follow-up that moves tests next to the code they cover.

It also adds `tui/src/app/test_support.rs` for shared fixture
construction.

This is just a mechanical refactoring (no functional changes) and does
not affect any production code.

codex-rs/tui/src/app/test_support.rs

@@ -0,0 +1,89 @@
+//! Shared App fixtures for app submodule unit tests.
+//!
+//! This module keeps heavyweight `App` construction and config-inspection helpers available to
+//! focused sibling test modules without making `app/tests.rs` the only practical place to test
+//! app-owned behavior.
+
+use super::*;
+use crate::chatwidget::tests::make_chatwidget_manual_with_sender;
+
+pub(super) async fn make_test_app() -> App {
+    let (chat_widget, app_event_tx, _rx, _op_rx) = make_chatwidget_manual_with_sender().await;
+    let config = chat_widget.config_ref().clone();
+    let file_search = FileSearchManager::new(config.cwd.to_path_buf(), app_event_tx.clone());
+    let model = crate::legacy_core::test_support::get_model_offline(config.model.as_deref());
+    let session_telemetry = test_session_telemetry(&config, model.as_str());
+
+    App {
+        model_catalog: chat_widget.model_catalog(),
+        session_telemetry,
+        app_event_tx,
+        chat_widget,
+        config,
+        active_profile: None,
+        cli_kv_overrides: Vec::new(),
+        harness_overrides: ConfigOverrides::default(),
+        runtime_approval_policy_override: None,
+        runtime_sandbox_policy_override: None,
+        file_search,
+        transcript_cells: Vec::new(),
+        overlay: None,
+        deferred_history_lines: Vec::new(),
+        has_emitted_history_lines: false,
+        enhanced_keys_supported: false,
+        commit_anim_running: Arc::new(AtomicBool::new(false)),
+        status_line_invalid_items_warned: Arc::new(AtomicBool::new(false)),
+        terminal_title_invalid_items_warned: Arc::new(AtomicBool::new(false)),
+        backtrack: BacktrackState::default(),
+        backtrack_render_pending: false,
+        feedback: codex_feedback::CodexFeedback::new(),
+        feedback_audience: FeedbackAudience::External,
+        environment_manager: Arc::new(EnvironmentManager::new(/*exec_server_url*/ None)),
+        remote_app_server_url: None,
+        remote_app_server_auth_token: None,
+        pending_update_action: None,
+        pending_shutdown_exit_thread_id: None,
+        windows_sandbox: WindowsSandboxState::default(),
+        thread_event_channels: HashMap::new(),
+        thread_event_listener_tasks: HashMap::new(),
+        agent_navigation: AgentNavigationState::default(),
+        side_threads: HashMap::new(),
+        active_thread_id: None,
+        active_thread_rx: None,
+        primary_thread_id: None,
+        last_subagent_backfill_attempt: None,
+        primary_session_configured: None,
+        pending_primary_events: VecDeque::new(),
+        pending_app_server_requests: PendingAppServerRequests::default(),
+        pending_plugin_enabled_writes: HashMap::new(),
+    }
+}
+
+fn test_session_telemetry(config: &Config, model: &str) -> SessionTelemetry {
+    let model_info = crate::legacy_core::test_support::construct_model_info_offline(model, config);
+    SessionTelemetry::new(
+        ThreadId::new(),
+        model,
+        model_info.slug.as_str(),
+        /*account_id*/ None,
+        /*account_email*/ None,
+        /*auth_mode*/ None,
+        "test_originator".to_string(),
+        /*log_user_prompts*/ false,
+        "test".to_string(),
+        SessionSource::Cli,
+    )
+}
+
+pub(super) fn app_enabled_in_effective_config(config: &Config, app_id: &str) -> Option<bool> {
+    config
+        .config_layer_stack
+        .effective_config()
+        .as_table()
+        .and_then(|table| table.get("apps"))
+        .and_then(TomlValue::as_table)
+        .and_then(|apps| apps.get(app_id))
+        .and_then(TomlValue::as_table)
+        .and_then(|app| app.get("enabled"))
+        .and_then(TomlValue::as_bool)
+}